Building an Extension: How to Plan, Budget and Manage Your Project

white kitchen diner extension with island and dining table
(Image credit: c/o KeDesign)

Building an extension isn't a one size fits all affair. Your build journey will be unique to you and your home so it's important to plan and manage it well for it to be successful.

Each extension will have its own criterion, whether you opt for a wrap around extension, a glazed single or two-storey extension, but there are some key details to get right, whatever your project. 

If you've not built an extension before how do you go from sketching your ideas down to seeing it come to fruition? Who do you talk to first? How should you budget? What do all would-be extenders need to know?

Our beginner’s guide breaks down all the areas you need to think about, from answering questions on planning permission and how much an extension costs, that will help kickstart your extension project.

Before You Start Building an Extension...


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Need more advice or inspiration for your project? Get two free tickets to the Homebuilding & Renovating Show

Before building an extension make sure you're aware of construction news locally and nationally that might affect your project - from materials to trades and changes in planning.

One area that's hitting the headlines is the construction material shortages and price hikes, and lack of HGV drivers that are having an impact on the build sector. Dwindling supplies of key building materials such as roof tiles, cement and a timber shortage are impacting the construction industry nationwide and prices have soared across several materials.

The shortage is expected to continue so this will have a knock on effect with the trades you want to employ (who are battling with backlogs from Covid, increase in demand and the material shortfalls) and with your budget, as inflated prices hit trades, who then have to pass that increase onto you, the client.

modern zinc clad extension to stone barn

(Image credit: Matthew Smith)

Factor in Extended Timescales

“With the material and labour markets still so volatile, clients just need to be prepared for delays,” says quantity surveyor Tim Phillips. “Lead times for quotes are only a little longer than normal due to the surge in demand,” says master builder Andy Stevens. “But start dates are way longer now. If you can get a good, experienced builder to start in under a year you’re doing well. You may be lucky if anything is cancelled due to planning issues, and so on, but I would aim for a year (obviously depending on where you are geographically).” 

And as far as build time goes? “Depending on build cost and complexity, I would allow five to six months as a minimum build time for small extensions, due to the current challenges in the industry,” says Darren Bray. “Most tender prices are coming back way over budget, so allow time to look at cost savings.” 

Many extension projects – big or small – are now choosing to use a quantity surveyor (QS) because of this to help keep a handle on costs as Tim Phillips explains: “More clients are aware of the nationwide price hikes, so projects where a client may have sought quotations from individual trades and project managed the build themselves are now seeking reassurance and advice from quantity surveyors.” So this might be something to bear in mind.

Be Realistic About Your Budget

It’s easy to daydream and get carried away with design ideas but one of the first steps to take is to be realistic about money. “Before taking the plunge it’s a good idea to work out roughly what size extension you can afford,” suggests Ian Rock MRICS, author of the Haynes Home Extension Manual and director of Most extension projects cost around £1,500- £2,000/m2 of new space and Ian agrees: “A ‘sticky note’ calculation of at least £2,000/m2 should give you a rough guide for build costs.”

And it's not just build costs to account for. “Making design decisions up front can help you keep in control of your budget so you know how much you’ll be spending on flooring, tiling, kitchen units etc,” says Jo Dyson Dyson, a partner at Mae House Design

Your extension will almost always cost more than you expected it to, so it’s important to have a contingency fund you can fall back on. “Allow a contingency of about 20%. There will always be unforeseen costs no matter how much you plan ahead in advance, especially with old buildings where there are a lot of unknowns,” says Jo.

How Much Does Building an Extension Cost?

Extension Cost Calculator

Try out our Extension Cost Calculator for an accurate estimate for your build project. 

How much does an extension cost? The million dollar question for any extender! 

Price hikes for materials, caused by the construction material shortage, and labour are occurring across the UK and will affect most bottom lines. Size, shape and glazing will also impact the cost of building an extension.

If you're carrying out a fairly standard single storey extension project, managed on your behalf by a builder, you should be looking to allow £1,800-£2,300/m2 for the finished scheme, with a two storey extension costing slightly less (on account of more cheaper space) at around £1,500-£2,000.

In certain parts of the South and high value areas of the UK - affected by labour shortages - this you could easily be paying £3,000/m2, particularly on smaller schemes. That makes an 8x4m kitchen extension a £50-£70,000 project; a two storey extension on the same footprint around £100,000. But, these figures are there to be broken depending on the chosen specification and the build route. 

building an extension - creating an entertaining space

(Image credit: Photographer John Selby, glazing by Schuco, fabricator Thomas Charles Grant Ltd)

Who Will Project Manage the Extension Build?

There isn’t a one size fits all when it comes to who will act as project manager. You can choose a build route that suits you, your budget and the type of extension you want to build.

Hire an architect

The first option is to have an architect or professional designer draw up plans for your scheme who can take you through the process until work gets underway on site. “Generally, if an architect is appointed for a full design service, we’ll assist the client in appointing and then co-ordinating the other specialists and consultants up to a start on site,” says Nimi Attanayake, founder and director at nimtim Architects.

This would encompass the co-ordination of the various consultants required before the project begins, including designers, structural engineers, party wall surveyors, the Building Control Officer plus any other specialists. Your architect will also see you through the planning permission process. 

Once construction begins, project management responsibilities could then be passed onto the main contractor. “This role would encompass appointing, co-ordinating and managing the separate construction packages and contractors, such as foundations, structure and finishes,” says Nimi. “The architect’s role during construction would then be one of contract administrator – facilitating the contract between the client (you) and the builder. We’d inspect the site regularly to ensure the project is being built according to our information.” 

Employ a Design and Build company

If you’d prefer to keep everything under one roof, there are design and build companies with in-house professionals who’ll be able to take you through from the start of the design process right up until completion. Or, for larger and more complex schemes, you might consider employing a professional project manager to oversee everything. Going down this route could save you time and potentially stress. Plus, using a professional PM brings the benefit of experience. They will be well-versed in managing the challenges that arise throughout the build because they’ll have done it plenty of times before. 

two oak frame side extensions to white cottage

(Image credit: c/o Welsh Oak Frame)

Project manage the scheme yourself

If you’re working to a tight budget, project managing the scheme yourself can be one area to save some pennies. As well as giving you a greater degree of control over the budget and construction, it could be an enormously satisfying experience. “Project managing a construction scheme is always challenging and difficult, but it comes with its own rewards and satisfactions,” says Nimi. “Providing you are comfortable enough to undertake it, the feeling of satisfaction and reward may be an advantage in itself.” 

Split the project management

It may be that you can split the responsibilities between yourself and your contractor. For instance, the building company could manage construction until the extension reaches watertight stage, at which point, you take over the role and project manage the rest of the sub-contractors (plumbers, electricians, plasterers etc.) yourself. Every project is different, so the key to managing your extension effectively is to come up with a strategy that maximises your skills, the expertise of the professionals you employ, and your budget.  

Factor in How Build Route Will Affect Costs

Which build route you choose will impact costs. Typical build costs based on routes are likely to be:

  • DIY: £300-£700/m2
  • Self-managed: £600-£1,400/m2
  • Main contractor: £1,000-£1,700/m2 
  • Design and build: £1,200-£2,000/m2 

building an extension

Glazing features heavily in this kitchen space, flooding the room with natural light. The Monrovia Kitchen featured, from Wood Works Brighton, starts from £35,000. (Image credit: Wood Works Brighton)

Talk to a Builder Early on

An informal chat with a couple of local builders could be a wise first move as, at this stage, you’re trying to get a sense of the feasibility of extending from an engineering and construction perspective. 

“A good local builder will likely have so much experience, extending all sorts of houses, that they have invaluable advice to impart, so use it to your advantage before getting plans drawn up,” recommends master builder Andy Stevens of Eclipse Property Solutions

“If you engage with a builder before plans are drawn up then you can discuss structural details, groundworks and so on first. Then you can take this information to your architect and plans can be drawn up accordingly.” 

If your home is in a sensitive location, for example, it might be worth getting a site appraisal by a planning consultant, too, who can advise on potential development with a site survey and feasibility studies. 

Here's what you need to know on how to find a builder.

Keep Costs Down: Can I Build an Extension Cheaply?

One way to keep costs down is choosing a more budget-friendly construction system. Concrete blockwork is readily available and most builders know how to work with the material, too. However, some modern methods of construction, such as structural insulated panels, may cost more to begin with, but may save in labour on site, especially when it comes to insulating your extension. 

Keeping the design and shape of your extension build simple — either square or rectangular shaped with a flat roof — will be less expensive than angled designs with more complicated roof structures and lots of glass. 

It is possible to design and build extensions for every budget, so putting in work at this stage can pay off later in the project. 

corten clad modern extension to terrace house

(Image credit: Simon Maxwell)

Building an Existing DIY

If you're asking 'can I build an extension myself?' the answer is to an extent. While chipping in and doing some of the work yourself can keep costs within budget, some jobs need to be left to the professionals (namely electrics). 

As chartered surveyor Ian Rock says, "Anyone with an aptitude for construction may relish the prospect of learning new skills, taking genuine pride in their achievements. But as a general rule it’s best to leave anything seriously time-critical to the professionals, along with all the heavy duty structural work. If your extension needs to be completed sooner rather than later it makes sense to stick to tackling jobs near the end of the project, such as landscaping." 

The type of work you choose to undertake when building an extension will depend on how happy and handy you are with the tools. Pick parts of the build to work on where there’s less risk of causing delays. 

Here are some DIY jobs that are most suited to DIY input…

laminate flooring in a kitchen

(Image credit: Flooring Superstore)

Internal joinery 

Timber stud partition walls are typically non-structural and should be a realistic project for the average DIYer to get to grips with. However, there’s usually time pressure to get them built so the electricians can start running first fix cabling. Installing door liners in the interior walls can also be worth doing, although these normally need to be in place before plasterboarding and plastering walls. Cutting and fixing skirting boards and architraves is reasonably straightforward, and can be left until the later stages, along with hanging interior doors and boxing in pipework. 


The most widely undertaken DIY flooring project is laying floor coverings. Types of flooring such as tiles, vinyl planks, or thin strips of interlocking laminate or engineered timber are relatively simple to install. However, installing structural timber flooring to the joists upstairs, such as traditional softwood floorboards or chipboard panels, should also be a feasible DIY project. To provide a temporary work platform for the builders, large sheets of oriented strand board (OSB) or plywood can be put in place to use as a deck. 

This allows you to postpone fitting the floor towards the end of the project, relieving time pressure and reducing the risk of pristine new floors getting damaged by building work or splashed with plaster. When it comes to the construction of ground floors however, these are not generally DIY-friendly because the insulation and floor screed work tends to be very time-critical and needs to be coordinated with other trades. 

building an extension and painting the walls

(Image credit: COAT)


Good quality decoration is key to achieving a professional finish, often ingeniously compensating for the less-than-perfect surfaces other trades leave in their wake. Nonetheless, deploying paintbrushes and rollers with a basic degree of competency is well within most people’s abilities, if you make sure you know how to paint a room before you start. Taking on the role of the decorator is also appealing from a DIY perspective because it isn’t usually too time critical, coming towards the tail end of the project. 


Wall and floor tiling are popular DIY projects that require dexterity combined with a logical mindset. Using good quality cutting tools is key to a successful outcome (specialist machines can be hired). It also greatly helps if you work to a clear plan and allow yourself plenty of time. As with all finishing trades you are to some extent at the mercy of those who have gone before; tiling onto perfectly true and level surfaces makes the job considerably easier! 

building an extension in matching materials

(Image credit: Thames Valley Window Company)

Use Modern Materials to Build an Extension

There are a handful of modern methods that self builders have been using that could well be adopted by extenders to potentially make the job of building an extension that bit quicker and easier.

Beam & block floor

Also known as ‘suspended concrete’, this floor structure has the advantage of being much less prone to structural problems caused by ground movement. Beam & block floors are more suited to large extensions where the existing house has a suspended timber floor that needs continuity of ventilation. 

Liquid Screed

Liquid floor screeds are much quicker to install by a specialist than conventional sand and cement screeds, but aren’t cost-effective for a small extension. If you’re adding UFH, then liquid screeds pair well. 


The cross-section gives them their name, but also means they can carry far heavier loads than a conventional timber joist of equivalent weight. 

Monocouche render

Monocouche house rendering can be sprayed or pumped onto exterior walls, which reduces the need for scaffolding and site costs. It’s low maintenance, durable and available in a variety of earthy colours. 

Prefabricated Roof trusses

Where extensions need to marry with existing roofs and walls, the need for customisation favours the traditional ‘cut timber’ route (cutting timber to size on site). However, if you’re building a large rectangular extension then prefabricated roof trusses (manufactured off site) can offer labour-saving advantages on site.

Will Building an Extension Add Value to My Home?

It’s all about balance. If you're likely to sell on eventually then you don't want to price yourself out of not recouping any of the money you spend on building an extension. 

Building a small house extension while adding value in terms of space it’ll create might not be that cost effective in how much it’ll add to the value of your house if you come to sell it. 

Adding a larger extension should increase the end value of your home but it’ll cost more to build so be mindful not to spend more than you’ll make back on it. Plus, there’s generally a ceiling price that you’ll want to keep within so make sure the numbers add up. 

Talking to local estate agents can help paint a picture of the local market and what extended homes similar to your own are selling for.

building an extension to create a dream kitchen

(Image credit: Wood Works Brighton)

What Size Extension Can I Build Without Planning Permission?

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Join The Planning Hub

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The Planning Hub is a new online resource that will help you understand how to get to grips with complex planning rules. Join today for access to easy-to-read guides which will provide you with key information to help you navigate the planning system.

You might find that you'll be able to build an extension under Permitted Development (PD), which means you won't have to go down the formal planning route. 

Do bear in mind that if your house is in a Conservation Area or a National Park, the amount of work under Permitted Development is usually reduced.

Under PD rights certain works can be carried out providing you meet certain criteria, such as:

  • You can extend a detached property by 8m to the rear if it’s a single-storey extension (6m for a semi or terraced house), or by 3m if it’s double
  • A single-storey extension can’t be higher than 4m on the ridge and the eaves, and ridge heights of any extension can’t be higher than the existing property
  • Two-storey extensions must not be closer than 7m to the rear boundary
  • Side extensions can only be single storey with a maximum height of 4m and a width no more than half of the original building
  • Any new extension must be built in the same or similar material to the existing dwelling
  • Extensions must not go forward of the building line of the original dwelling
  • In designated areas (such as areas of outstanding natural beauty, conservation areas, etc), side extensions require planning permission and all rear extensions must be single storey
  • An extension must not result in more than half the garden being covered

building an extension in oak

(Image credit: Arboreta)

From 31st August 2020, the rules changed so that two-storey extensions on detached, semi-detached and terraced houses will be fast-tracked as long as they get prior approval. This means the local authority have to be notified of the details before the project starts and it’s a much more involved process with the Local Authority. 

There are some restrictions an extension has to adhere to:

  • Once works have been completed, the building must not be more than 18m high (excluding plants)
  • The floor-to-ceiling height of any additional storey must not be more than 3m in height or higher than the floor-to-ceiling height of any of the existing storeys
  • The overall height of the extension, including the roof, must not be more than 7m high.

If you’re planning on building a large extension then it will, more than likely, need planning permission and you will need to submit an application. 

It's wise to engage with your local authority early on and research local planning policies to know what’s likely to get approved before you submit an application.

You can submit an application via the Planning Portal. An application in England for an extension currently costs £206.

Do I Need a Lawful Development Certificate?

Even if your extension project can be done under PD rights, it’s worth applying for a Lawful Development Certificate (LDC) from your local authority to confirm that the work was lawful and met PD requirements and didn’t need planning permission. This is well worth doing if you plan to move, too. 

It costs £103, half the normal planning fee.

How Close to a Boundary Wall Can I Build an Extension?

If building your extension involves digging or building foundations within 3m of the boundary, party wall or party wall structure, or digging foundations within 6m of a boundary, the work will require you to comply with the Party Wall Act (see more below).

Building an Extension to the rear of the house

The homeowners of this detached home wanted a better and more seamless connection between house and garden so built a contemporary side extension garden room. The glazing system was supplied by Thames Valley Window Company. (Image credit: Thames Valley Window Company)

Does the Party Wall Act Apply When Building an Extension?

Good to know

You don’t need to have planning approval to serve a Party Wall notice. They are separate. 

There are instances when the Party Wall Act applies to building an extension. Chartered surveyor, Ian Rock explains: 

“For aspiring extension builders the part of the Act that’s often most relevant is where it applies to the excavation of foundations close to neighbouring buildings or garden boundary walls. In order to trigger this legal minefield, excavation normally needs to be within a critical distance of three metres from the adjoining property where your new trench is deeper than their existing foundations. Because older properties tend to have relatively shallow footings in most cases it’s a ‘given’ that the new ones supporting the extension will be considerably deeper. 

"Where there is any doubt, it might be worth consulting any records of foundation depths or seeking an expert opinion, for example from a Building Control surveyor (since the neighbours may not relish the prospect of trial holes being dug next to their home to confirm actual foundation depths). 

"Be warned, however, that in some cases excavating within six metres of an adjoining property can also be covered by the Act. But this only applies where the new foundations are so deep that drawing an imaginary line downwards at a 45° angle from the bottom of the next door’s foundations would hit them, for example on a steeply sloping site or where you’re incorporating a basement. 

"There are two other situations where the Party Wall Act often applies. It’s not unusual for new extensions to be designed to maximise floor area by building right up to, or ‘astride’ the garden boundary between two properties. The second situation would be in cases where you physically cut or alter a party wall, such as where you want to build onto your neighbour’s existing wall so it becomes your new extension’s party wall. 

"Happily, minor works like fixing screws or plastering onto party walls are not significant enough to be covered by the Act, but resting a new beam within the wall, as you might for a loft conversion, definitely would be.”

Learn How to Plan the Build

What to Consider Before Drawing up Plans

There are some important aspects to consider before you get to the stage of getting your plans drawn like:

Importantly, notify your insurer of the work. Some may not provide cover during the works, but others offer dedicated extension insurance products.

building an extension in the loft and on the ground floor

This London townhouse had both a loft and rear extension, designed by The Gentleman Architect, to transform how the house functioned as a home. (Image credit: The Gentleman Architect)

Should I Employ an Architect when Building an Extension?

Useful Contacts

Most of these bodies require full members to have relevant academic and technical qualifications. Whichever designer you choose, ensure that they carry sufficient professional indemnity insurance.

You've got house extension ideas in mind but now you need someone to turn that sketchy line drawing into a plan. 

You can opt for a build and design company, or you can choose an architect or architectural technologist. However, don’t just approach the biggest or most well-known architectural practices in your area. 

Look around and “choose an architect who you feel really gets your lifestyle and family life, as well as what you want, and don’t want, from an extension,” suggests architect Darren Bray of Studio Bad.

If you're competent with CAD then you can draw up plans yourself, although more complicated designs will benefit from a more expert eye due to the complexities. 

Bear in mind, if you’ve designed the extension yourself you may need the aid of a designer or draftsperson to draw up your plans to submit them for planning approval (if required) and a structural engineer to produce drawings and calculations for Building Regs’ purposes. These will also form part of the tender documents when hiring builders.

large modern extension to brick cottage

(Image credit: Simon Maxwell)

A Design Brief From You is Key

“Whether your first meeting with an architect or designer is a formal design consultation, where the first sketch design proposals are drawn up or it’s just an initial chat, it’s best to write up a short design brief detailing how your home works or doesn’t work for you,” advises Laura Jane Clark, founder of Lamp Architects. “It will then help form the basis of the first discussions you have with an architect.” Plus include any prior knowledge you have gleaned from builders or surveyors about the site. 

Be clear, too, about your budget as this will help a designer create a design brief that’s achievable and desirable for you. “This will signal to your architect that you are serious about getting project work done, and it encourages them to address this key item at the beginning of the process,” advises founder of Gbolade Design Studio, Tara Gbolade. 

Do I Need to Comply with Building Regulations When Building an Extension?

Whether you require planning consent or not, you will need to make sure you comply with Building Regulations. To meet the Regs, you can either submit a Full Plan Submission or a Building Notice.

  • Full Plan Submission: send plans to your local authority building control or approved inspector prior to the build for approval. The building inspector visits your site at different stages and inspects the work as it progresses
  • Building Notice: a statement which lets the local authority know that you will be complying with the regulations in building your extension and gives the building control department 48-hours notice of your intention to start the work. Building inspectors will inspect the work at various stages and will advise you of any problems

A Building Notice is the riskier of the two as you may only find out you have a compliance issue once building work has started, which then needs to be paid to be put right.

All alterations to listed buildings, including internal ones, require consent and it is a criminal offence to alter a listed building without it.

With a listed building the planners will always regard the existing property as more important than what you are proposing to add to it. Any extension will therefore have to respect the flavour, appearance and historic material used in the construction of the original house.

Getting a warranty for your extension is advisable as it will cover you for things such as structural defects or faulty workmanship. Should the worst happen, the builders who did the work will need to return to put right what has gone wrong.

duct grey kitchen from Trend Interiors with rooflights

(Image credit: Trend Interiors)

How to Finance an Extension Project

There are several options when it comes to borrowing money to finance your extension.

In an ideal world, we would all finance our projects using our savings, but if you need to borrow the money, your best options are:

Credit card: If you need to borrow thousands rather than tens of thousands for your work, an interest-free credit card is a good choice. Shop around — the best deals offer up to 27 months of zero-rated spending

Loan: Personal loans of up to around £25,000 are suited to smaller projects. The loan may be enough to cover the building work and if you are fitting kitchens or bathrooms in your extension you can often buy these on finance from larger stores. Just keep an eye on repayments as they can quickly add up when you borrow from several sources

Remortgage or secured loan: If you are borrowing more than £25,000 you may need to remortgage or take our a secured loan against your home. Many building societies offer a Home Improvement Loan of up to £200,000.

building an extension that celebrates the existing building

This stunning kitchen extension, designed by The Gentleman Architect, embraces the original house leaving brickwork exposed and highlights the new with the striking geometric-style rooflights that stitch the buildings together. (Image credit: The Gentleman Architect)

Your builder will generally stage payments for each phase of the project. Try not to make any payments upfront.

If it is necessary to purchase an expensive item perhaps a bathroom suite some months in advance of its installation, you should make sure it is in your name and not that of the builder, just in case they should go out of business.

Your designer will charge fees according to the work involved, so an accurate quotation of the fee would be a very useful thing to have before you give the go-ahead for plans to be prepared.

It is wise to check what any quote includes and who will pay any local authority application fees. Also, ask if any provision has been made should structural calculations be required.

Can I Reclaim VAT When Building an Extension?

VAT is zero-rated on new build in domestic properties including self builds, but this does not include extensions.

If you are renovating or extending a property that you can prove has been empty for 2 years or more, it will be treated as a conversion and therefore be charged at the reduced rate of 5%. The VAT concessions are only available via a VAT-registered contractor.

How to Prepare Tender Documents

“The aim of going out to tender for works is to obtain a fair and competitive price from the market to achieve the project you are proposing,” says project manager and chartered surveyor Bob Branscombe. 

Approaching how to prepare tender documents can be tricky. “First, approach at least four (ideally six) builders informally, to brief them about your scheme and to gauge interest. Then whittle the list down — you’ll probably have three to four companies who have agreed to tender. These will form your list, and will all be sent a pack of documentation."

The pack must contain a framework to cost the job, a timescale to price the work and a copy of the legislative framework, plus key documents such as drawings, specifications, planning permission compliance, and so on. Give them a timescale to get back to you by, but be realistic - give them at least four weeks - there’s a lot for them to go through. 

When you get the bids back, go through them thoroughly (your QS, if you have one, can look over the cost breakdown too), and ask questions. Don’t be afraid to push back and query items you’re not sure about. Once you’re happy with your chosen builder, you’re ready to start your build!

Ian Rock

Chartered surveyor Ian Rock MRICS is a director is and the author of eight popular Haynes House Manuals, including the Home Extension Manual, the Self Build Manual and Period Property Manual.

Ian is also the founder of Zennor Consultants. In addition to providing house surveys, Zennor Consultants provide professional guidance on property refurbishment and maintenance as well as advising on the design and construction of home extensions and loft conversions, including planning and Building Regulations compliance.

Ian has recently added a 100m2 extension to his home; he designed and project managed the build and completed much of the interior fit-out on a DIY basis.