Side extensions: Expert advice for building and planning a sideways move

white cottage with grey clad modern side extension
(Image credit: Charlie O'Beirne)

Side extensions can be a fantastic way to increase the footprint of an existing house, often without taking up too much garden space. They can also transform the kerb appeal of a home, particularly when combined with recladding the existing house.

There are many factors which can influence where to build your new house extension. From budget and space, to personal taste and the existing house — every home improvement project is unique. 

When choosing to extend to the side of a house, planning permission, Party Wall Agreements and boundary lines all come into play. Find out what you should consider, the potential costs and how to plan your side extension project. 

What is a side extension? 

Side extensions sit to either the right or left hand side of an existing house — this is often to preserve a garden or to fill in an unused side return. 

"Utilising the side of your house can add real value to a house. Not only for re-sale but also floor area," explains Adam Knibb, founder of Adam Knibb Architects. "There are numerous houses that were designed in a very linear form; this can create quite a limited floor plan. Extending to the side of the property (whether at ground or first floor) can drastically increase the enjoyment of the house."

Building an extension to the side can also dramatically change the appearance of a house, either to solve imbalanced architecture of the 60s and 70s, or to modernise it with the introduction of cladding or other finishes. 

"We [Adam Knibb Architects] like to use contemporary forms which allow us to get the most out of any useable space but also to create a clear definition between the old house and proposals," adds Adam.

semi detached house with modern side extensions

This semi detached house was extended into the awkward side garden; the extension, designed by Selencky///Parsons, has created larger living areas and additional bedrooms.  (Image credit: Chris Snook)

Do you need planning permission for a side extension? 

Many homeowners can add single and double storey extensions to their homes under permitted development – which means you do not need to apply for planning permission – but there are caveats, particularly when it comes to adding a side extension.

"While permitted development allows for some side extensions — this can be limited (especially if looking to go two storey)," advises Adam Knibb. "You will almost always need planning permission if you want to change the visual look of the side extension compared to the existing house."

"It’s worth noting that the planning permission rules around double storey extensions are more complex," adds Richard Morgan, design director of Resi. "So, we highly advise working with a designer and securing yourself a lawful development certificate to avoid falling foul of the scheme. 

"You should also check Article 4 isn’t in effect in your area, this is when your local authority suspends permitted development rights and is usually in effect in and around conservation areas." 

Should you hope to extend under permitted development, caveats include:

  • The extension cannot sit forward of the principal elevation
  • It should be built using similar materials
  • If it is within 2m of a boundary, the eaves cannot be higher than 3m and not taller than 4m otherwise
  • The width of the side extension must not be greater than half the width of the original dwelling

If your house extension ideas go beyond of this criteria, then you will need approval from the local planning department. 

But, if your side addition will utilise small house extension ideas, the work could still fall under permitted development. If in doubt, always check with your local authority before starting any building work. You can also see the planningportal.gov.uk for a full list of caveats.

Regardless of whether you need planning permission, you will need Building Regulations approval. “Your extension will need to comply with the Building Regulations," advises homes journalist Rebecca Foster. "In order to do this, it’s best to submit a Full Plans application — commissioning and submitting Building Regs plans that show the building control inspector how to extension conforms prior to the work commencing."

How close to my boundary can I build a side extension? 

This may depend on how close to neighbouring homes your new side extension will be.

"Side extensions can mean that you will be looking to undertake ground works that might fall within specific Party Wall foundation codes," says Adam Knibb. "Always ask a Party Wall surveyor to review the works and advise if any notices need to be served."

"Building work involving work the boundary wall between your house and your neighbours needs to comply with the Party Wall Act and, as part of the process, you’ll need to serve a party wall notice to adjoining neighbours."

How much does it cost to build a side extension? 

Scott Joyce, director of Scott Joyce Builders, estimates "a guide price of £2,250-£2,500/m2 depending on where you are in the country" for a side extension. "A double extension would still work out the m2 price (so effectively double)," he says.

"Other contributing factors include project size, complexity, the quality of materials and your selected contractor," says Resi's Richard Morgan of extension costs. "If someone wants to keep costs to a minimum, I always recommend trying to make savings in terms of fittings, rather than cut corners in terms of the build. 

"Invest in a good builder and good design, as these are costs you’ll most likely recoup from when it comes to selling in the future, whereas things like kitchen fittings will depreciate in value over time."

How can a side extension work with a terrace house? 

A side extension is a possibility for an end-of-terrace home, should there be sufficient space (typically in the form of a side garden) to the side of the property.

Although it might sound odd, even a mid-terrace can undergo a side extension of sorts. This is generally added to the rear section, known as the outrigger, which often houses the kitchen or downstairs bathroom. 

The usually underused space to the side of the outrigger is called a side return and by adding a side return extension within this section a more usable layout can be created. Often, homeowners combine this with an existing space, or add a larger rear extension, to create an open-plan kitchen diner or living space overlooking the garden.

How can a side extension improve a semi-detached property? 

For semi-detached houses, side extensions are a smart option. Rear extensions can impact neighbours' natural light and cause some issues while front additions may disrupt the symmetrical aesthetic for the pair (and almost certainly need planning permission). 

Rather than one room made bigger or added to the layout, side extensions to semis can make multiple living spaces larger, making a bigger impact to the floorplan as a whole. 

Can you add a side extension to a listed house? 

Given that you have obtained Listed Building Consent and planning permission, listed building can be extended with a side extension. 

Many owners of the historical buildings (and often the local planning department) enjoy more contemporary-styled side extensions as there is a clear distinction between the new architecture and the old. 

stone and brick house with timber clad side extension and gravel driveway

Adam Knibb Architects added a modern timber frame extension to the side of this brick and stone Grade II Listed House to create an L shape courtyard layout. (Image credit: James Marris c/o Adam Knibb Architects)

Can you extend over a side garage?

For those with an attached garage to the side of a house, a conversion and/or extension above is a natural choice — and can be used to add a further bedroom, bathroom or study at first floor level. As long as the foundations for the existing garage are sufficient, extending this way will be a cheaper option than building entirely from scratch. A structural engineer will be able to aid here.

If the existing foundations are not sufficient to support an additional storey, then there are engineering solutions which can be employed to strengthen them and the existing garage. However, the cost of this work will need to be weighed up against the cost of knocking down the existing garage and rebuilding a two-storey extension. The latter may also provide opportunity for a wider extension, depending on the size of your plot.

Our in-depth guide to extending over a garage provides a full run-down of the factors and practical implications to consider.

white rendered house with side extension over garage

Extending over the existing garage and rendering the entire front of the property has completely transformed this 1980s estate house.  (Image credit: Jeremy Phillips)

Is it worth doing a side extension?

"In almost all circumstances the addition of a well created side extension will add value to a house," says Adam Knibb. "Obviously this needs to be balanced against the cost to construct the extension. 

"We find that a useful stepping stone is to ask a local estate agent to quickly review our proposals to give an idea of uplift for the client. They can then weigh this up against the construction costs to make sure it all stacks up."

Amy Reeves

Assistant Editor Amy began working for Homebuilding & Renovating in 2018. She has an interest in sustainable building methods and always has her eye on the latest design ideas. Amy has interviewed countless self builders, renovators and extenders about their experiences for Homebuilding & Renovating magazine. She is currently renovating a mid-century home, together with her partner, on a DIY basis, and has recently fitted her own kitchen.