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20 Things You Need to Know Before Extending Your Home

All the information you need to get your extension off to a great start

Adding living space is more popular than ever, but many people start their extension project without knowing enough about design, the law, construction and planning. We reveal the 20 things you must know - and probably don't.

The ‘Right to Light’ Exists in Law

Your neighbour may attempt to block your plans by claiming they have a legal right to light to one or more of their windows. There is such a thing as a legally established ‘right to light’, usually estab­­lished auto­matically ‘by prescription’ after 20 years, however, it is only relevant in limited circum­stances.

A right to light is a type of easement and overrides any planning permission you might have and your permitted develop­ment rights. It can in theory, therefore, prevent you from blocking out a neighbour’s window. How­ever, it only provides for whatever light is reasonably required for the use of the building. It does not mean that your exten­sion cannot obstruct a neighbour’s window or view, or reduce the amount of sunlight entering – these are planning consid­er­ations.

Rights of light are only likely to be relevant in city centres where buildings are very close together. In such circumstances contact a specialist lawyer.

You Can Have WCs Anywhere

It used to be a requirement of the Building Regulations for there to be a lobby between a WC and any other room. This was commonly conceived to be for reasons of hygiene and to relate to the kitchen where food is prepared.

Although it is desirable for reasons of privacy to locate WC facilities off hallways, circulation space, lobbies or the utility room, this is no longer necessary under the Building Regula­tions. It is, however, necessary for there to be a wash basin and suitable ventilation to all WCs.

(MORE: Building Regulations Guide)

Minimum Ceiling Heights

Although the legal minimum ceiling height has now been removed from the Building Regulations, there is still a practical minimum height and this is especially worth thinking about in attic and cellar conversions. All rooms should normally have a floor to ceiling height of at least 2.1m throughout (standard ceiling height is 2.4m). In rooms with sloping ceilings, at least 50% of the floor area should normally have a floor to ceiling height of at least 2.1m.

Avoid Through Rooms

Working out the most efficient and practical way to access an extension is often the great­est design challenge. Do not sacrifice more than you are gaining, for instance by slicing up a good sized bedroom in order to gain access to an extension that adds only one more bedroom of a similar size.

Using an existing room to access an extension rarely works unless it is sufficiently large and the furniture carefully arranged. Such rooms usually end up as nothing more than a corridor and a dumping ground for home­less storage.

Circulation space is very important to the healthy function of a house, ensuring that it is liveable and that the best use is made out of all of the space. When extending, this may mean rethinking the function of every room in order that the principle rooms, most importantly the kitchen, dining and living space, can all be accessed directly from the main hallway.

In smaller houses where there is no space for a separate hallway, it is a good idea to have at least a small lobby or enclosed porch to create privacy from the front door.

Make Your Conservatory Part of Your Home

You can integrate a conservatory into the existing house to make it an extension to an existing room, rather than a bolt on, but you have to be careful with the design.

The Building Regulations require most conservatories to be separated from the existing house by exterior quality doors. Such a doorway with a threshold can leave the new space feeling isolated from the rest of the house and unless the conservatory is large enough to work as a room in its own right (the minimum is around 3m x 4m) it can end up being an expensive, underused space.

Double doors can be left open to help make a conservatory feel like part of the house, but even double glazed conservatories lose heat quickly and so most people end up closing their conservatory off for the winter months to help keep their home warm and their fuel bills down.

With a bit of redesign to reduce the glazed area, the section of exterior wall separating it from the existing house can be completely removed. This turns the conservatory into a true extension, and by incorporating sections of plastered wall and insulated solid roof, a conservatory can be used to extend an existing room such as a dining room or kitchen.

To turn a conservatory into an extension you must provide your local building control department with calculations that show that the amount of glazing in the windows, doors and roof of the conservatory/extension, together with the amount of glazing in the windows, doors and any rooflights in the original house, do not exceed 25% of the floor area of the conservatory and all floors of the house added together.

New windows and doors in the conservatory/extension will need to meet the current U-values required by the Building Regulations.

(MORE: Glazed Add-ons Design Guide)

Consider Adding Basement Space

If you have an existing cellar, you can convert it into living space without using up the volume allocated to you under per­mitted develop­ment rights. Creating basement windows and external access will not usually require planning per­mis­sion either, although it is always worth checking your local authority’s policy on basements. All work must, however, comply with the Building Regula­tions laid out in the Approved Document – Basements for Dwellings 2000 (www.basements.org.uk)

Converting existing cellar space to bring it up to habitable standards costs from £1,000-1,500/m² providing there is already enough headroom. Creating a new basement beneath an existing building to add extra space is also possible. The cost is £3,000-4,000/m². Due to the cost, it is usually only financially viable to retrospectively add a basement in high value areas such as Central London.

(MORE: Converting a Basement)

Balance Your Accommodation

Although there are no legal requirements to provide more than one bathroom, it is practical to have a least one full bathroom on the same floor as the main bedrooms. For larger households it helps to have at least one en suite bathroom — ideally to the master bedroom.

If you are extending to add extra bed­rooms, clearly creating the number of bed­rooms needed for the house­hold is the main priority, but this should, if at all possible, be balanced by an increase in the number of bath­rooms. Future buyers will expect at least one bathroom and a shower room on a four or five bedroom house and without this the value will be constrained.

An additional bathroom on the ground floor of a house should be a last resort. It is functional but will add little if anything to the property’s value as bathrooms away from bedrooms are impractical (aside from for those who work outdoors).

Know the Building Regulations

Even if you do not need planning permission for your extension, because you are using permitted development rights, you must get building regulation approval.

The Building Regulations set out minimum requirements for structural integrity, fire safety, energy efficiency, damp proofing, ventilation and other key aspects that ensure a building is safe.

Most repair work is excluded from the Building Regulations, with the exceptions of replacement windows, under­­pinning and rewiring. However, apart from certain new buildings such as sheds, outbuildings and some conser­vatories, all new building work, including altera­tions, must comply with the Building Regulations.

Typical Examples of Work Needing Approval:

  1. Home extensions such as for a kitchen, bedroom, lounge, etc.
  2. Loft conversions. Internal structural alterations, such as the removal of a load-bearing wall.
  3. Installation of baths, showers, WCs which involve new drainage or waste plumbing.
  4. Installation of new heating appliances. 
  5. New chimneys or flues.
  6. Altered openings for new windows.

You Can Start Within 48 Hours of Notifying Building Control

Once you have dealt with planning, if you are in a hurry to start building your exten­­sion, you can commence work immediately after giving the local authority building control depar­­tment 48 hours’ notice. You are required to submit a ‘Building Notice’ and the required fee.

Generally the Buildi­ng Notice method is more suitable for simple works where detailed draw­ings are not required, but it can be used for any project, with the exception of work to listed buildings.

For most extensions it is best to make a Full Plans application. This involves submitting detailed drawings, specifications, calculations and a location plan for inspection by the local authority, together with the appli­ca­tion forms and appropriate fee.

Building control has to respond within five weeks unless you agree to give them an extension to two months. A Full Plans submission allows any irregularities to be resolved before work commences.

With a Building Notice, the building control officers can ask for proof of compliance at any stage, so it is essential to make sure they make all necessary inspec­tions and provide any structural calcula­tions when requested. When the project is com­pleted and inspect­ed by the local authority, a completion certifi­cate will be issued which will prove useful if the property is ever to be sold on. Application fees are set individually by each local author­ity.

(MORE: What You Can and Can't Do to a Listed Building)

Remove Your Walls

You don’t need walls to define rooms. Creating larger, more open spaces will help to make a prop­erly feel larger. The fewer walls you use, the more spa­cious and light a property will feel.

You can define separate rooms and functions by using all sorts of other features such as furniture, lighting, floor coverings, decoration, floor or ceiling levels, and informal room dividers such as kitchen island or peninsula units, fire­places, open shelving, island walls or even the stair­case.

If you are unsure whether it will work, visit show houses built by developers. Most now feature at least an open plan kitchen breakfast space instead of a separate kitchen and dining room.

(MORE: Open Plan Living)

There are Different Rules For Conservation Areas

If you live in a Conservation Area your Permit­ted Development (PD) rights – extensions and alterations that do not require planning per­mis­sion – are restricted. Each local authority has its own policy for Conservation Areas but generally the basis of the policy is to prevent the loss of character of the Conservation Area.

Consequently planning is required for the addition of dormer windows or any other change to the shape or height of the existing roof, including the addition of Velux rooflights if they face the highway. This means that attic conversions will almost always require planning permission in a Conservation Area.

(MORE: Dormer Windows)

Have You Really Thought Through Your Loft Conversion?

Before embarking on an expensive attic con­ver­sion think carefully about the cost relative to the amount of useful space that can be gained, and the impact on the existing accom­­modation. In order to comply with the Building Regulations to form an additional bedroom in the roof space, the floor may need strength­ening and the roof will need to have at least 150mm of insulation, plus a 50mm clear air gap (unless you are able to replace the existing roofing felt with a breathable mem­brane). 

The result of bringing the loft space up to habitable requirements is that the ceiling height will typically be lowered by 60-100mm and the floor level raised by at least 15-20mm. Both of these changes will reduce the amount of usable space with good headroom. If steel purlins or joists need to be added, too, this can further reduce the space.

If the usable space is limited, consider raising the roof height by rebuilding it, or even lowering the existing ceiling height below. Accessing a habitable loft conversion requires a permanent staircase and this will need landing space.

If you decide the cost of a full conversion cannot be justified because of space constraints, con­sid­er improving the loft space to create storage accessed by a fold-away loft ladder or space-saver stair­case? This can be achieved at a signif­icantly lower cost than a full conversion. Attic conver­sions cost from £600/m² for a simple con­version, up to £1,500/m² for more compli­cated work.

(MORE: Loft Conversion Design Guide)

Extending a Listed Building Can Be Problematic

Permitted Development rights do not apply to listed buildings, so any extensions will need both planning and listed building consent. The design of any extensions to a listed building need to be of a very high quality and it will be necessary to use the services of a specialist architect or surveyor.

It will be essential to work closely with the local authority conser­vation officer. Each officer will have their own perspective on what will alter the character of a listed building. It is a criminal offence to alter a listed building, inside or out, without listed building consent.

Permitted Development is Not Always Straightforward

Under the new rules, the ‘original’ (as it stood in or prior to 1948) rear wall of a detached home can be extended by up to 8m in depth with a single storey extension; this is reduced to 6m if you live in a semi or terrace. If your proposed new extension will be within 3m of a boundary, then the eaves height is limited to 2m under PD.

If you hope to build a two storey extension (no higher than the house), this can project up to 3m from the original rear wall, so long as it is at least 7m from the rear boundary. It’s also important to note that no extension can project beyond or be added to what is deemed to be the front of the house or an elevation which affronts the highway. A side extension cannot make up more than half your house’s width.

Furthermore, with the exception of conservatories, new extensions must be built of materials ‘similar in appearance’ and with the same roof pitch as the main house. So while PD rights are beneficial, there’s a lot to consider before starting work.

Is there anything else I can do under my PD rights?

Prior to the new legislation, volume limitations were applied to the entire house — so if you extended, you were unlikely to be able to convert your loft under PD rights as well. The good news is that the latter has now been separated out, allowing you to undertake both without one restricting the other. So, you can also convert your loft into a bedroom or extra living space by up to 50m³ in a detached house, or by 40m³ within any other home.

(MORE: Permitted Development)

Minimum Room Sizes

It can be tempting to try and subdivide existing and new space into as many bedrooms as required, particularly if budget or the size of the extension permit­ted is restricted. However, there are minimum sizes beyond which rooms will not function.

When considering applications for conver­sions, most local authorities have recom­men­ded minimum room sizes which planning applica­tions must conform to. However, the rules about sizes are more applicable to social housing and are usually relaxed for private accommodation.

Function of room Minimum Size
Double bedroom 10.2-11.2m²
Single bedroom 6.5-8.4m²
Living room 13-13.2m²
Dining room 9.5m²
Kitchen 7.2-7.65m²
Kitchen/diner 12m²
Lounge/diner 15-18m²
Bathroom and WC 3.6m²
Bathroom only 2.8m² (or 1m² of circ.)
WC only 1.3m²
Hallways and landings 900mm width

In addition, rooms must always have external windows, with the exception of non-habitable rooms such as kitchens, bathrooms, dining rooms and studies.

Know the Party Wall Act

Your neighbours cannot stop you from build­ing up to, or even on, the boundary between your properties, even if it requires access onto their land (providing you have planning permission to do so, and there are no restrictive covenants).

The Party Wall Act etc. 1996 allows you to carry out work on, or up to, your neighbours’ land and buildings, formalising the arrange­ments while also protecting everyone’s inter­ests. This is not a matter covered by planning or building control.

If your extension involves building or digging 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. In these cases you may need a surveyor to act on your behalf. The act does not apply in Scotland.

(MORE: The Party Wall Act)

There is a Difference Between an Estimate and a Quotation

An estimate is normally a contractor’s guess as to what your extension will cost. Whether given verbally, or in writing, is not legally binding and the final bill may exceed it.

A quotation is a definite price. When deciding which builder to choose, always get written quotations from at least two firms, ideally ones that have been recommended to you.

The written quotes should itemise the work to be done, provide a breakdown of costs and a total, and state whether VAT is included. When you receive the bids, check whether there are any caveats which might involve extra expense. Also, compare provisional sums for work such as foundations to make sure you are comparing like with like.

(MORE: Quotes & Estimates)

(MORE: Comparing Quotes)

Beware of Removing Trees

Some trees are protected by Tree Preservation Orders (TPOs). Even if an extension does not require planning permission you cannot alter or even prune a tree that has a TPO on it without planning permission.

All trees within a Conservation Area are protected by legislation and effectively have a TPO on them providing they have a trunk of diameter greater than 75mm. Altering a tree that is protected by a TPO is a criminal offence and can result in substantial fines.

(MORE: Tree Preservation Orders)

A Shower Room Can Be Added Anywhere

A basic shower room need measure only 900mm wide by 1.8m (or 2.6m if it is to include a WC and basin). The cost of creating a shower room within a bedroom will be £2,500-3,000 providing the plumbing is within reasonable proximity.

You Probably Can’t Claim VAT Relief

Most extensions will be subject to VAT on labour and materials at the standard rate of 20%, especially if you use a contractor to undertake the work. If you use local trade­smen who are not VAT registered you can save the 20% VAT on their labour, but you will still have to pay VAT on materials at the standard rate.

Some extension projects are eligible for VAT relief, such as work to listed buildings (zero rated), the conversion of an existing dwelling that changes the number of units (reduced rate of 5%) and work to a building that has been unoccupied for at least two years (reduced rate of 5%).

To benefit from VAT relief you must use a VAT registered builder — you cannot reclaim the VAT yourself.

(MORE: Reclaiming VAT)

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