Building a house extension: What to know before you begin

Adding an extension can be a smart way to improve your home without the hassle of moving. It gives you more space, more light, and often more value too.

But before the builders arrive, there’s a lot to think about. From planning permission to choosing the right layout, getting it right at the start will save time, money and stress later on.

What to consider before you start

Before making any big decisions, take time to weigh up your options. Ask yourself:

• What type of extension suits your property best?
• Do you need planning permission, or will permitted development rules cover it?
• How much will it cost – and will it add value in the long run?
• Are there any issues with access, neighbours or design restrictions?

The more information you can gather early on, the easier the process will be once you speak to a designer, builder or architect.

Types of house extension

Each property is different, but here are the most common extension styles in the UK:

Rear extension

A popular choice for semi-detached or detached homes. It extends into the back garden, usually to add a bigger kitchen or open-plan living space.

Side return extension

Great for Victorian or Edwardian terraces with an unused alleyway at the side. It widens the ground floor without eating into the back garden.

Wraparound extension

A combination of rear and side return extensions. More ambitious, but gives you a large open-plan area with flexible layout options.

Single-storey extension

Ideal for sunrooms, home offices or utility spaces. A simpler project than going up a level, and more likely to fall within permitted development rules.

Double-storey extension

Builds out on both the ground and first floors. It can transform your home’s layout and value but will usually require full planning permission.

Loft or basement conversion

Technically still an extension – just going up or down instead of out.

Do you need planning permission?

Some extensions can be built under permitted development rights, which means you don’t need full planning permission. But there are rules:

• Rear extensions can only go so far: 3m for a terraced house, 4m for a detached
• The materials and appearance must match the existing house
• Single-storey extensions must be under 4m high
• Two-storey extensions need to be further from the boundary
• No balconies, verandas or raised platforms are allowed under permitted development

If your home is in a conservation area or is listed, the rules are stricter. Always double-check with your local authority before you begin.

Building regulations still apply

Even if you don’t need planning permission, you’ll still need to comply with building regulations.

These cover everything from structural safety and insulation to fire safety and ventilation. You’ll need technical drawings (usually from an architect or designer) and approval from building control.

Once the work’s finished, you’ll be given a completion certificate to confirm everything meets the standards.

Party Wall Act: what you need to know

If your extension is close to a neighbour’s property or shares a wall, you may need to issue a Party Wall Notice. This applies if you’re:

• Building on or near the boundary
• Carrying out work on a shared wall
• Excavating within 3 to 6 metres of a neighbouring property

If your neighbour agrees in writing, you’re good to go. If they don’t respond or object, you may need to appoint a surveyor to create a Party Wall Award. You can find full details on the gov.uk Party Wall Act page.

Insurance during building work

Before work starts, tell your home insurance provider. Most standard policies don’t cover damage or problems that happen during building work.

It’s also worth checking that your builder or contractor has public liability, employer’s liability and professional indemnity insurance.

If you’re moving out during the project, you may need unoccupied property insurance as well.

Thinking about energy efficiency

If you’re investing in an extension, it’s worth thinking about efficiency early on.

Better insulation, modern heating and natural lighting can help reduce long-term running costs. Plus, it makes your home more appealing if you ever sell or remortgage.

What about access?

Logistics matter. Think about how builders will get in and out, where materials will go and whether the work will affect parking or nearby paths.

Some councils may ask for extra documents if access is tricky or if work affects the pavement or road. It’s best to sort this out before planning begins.

Should you manage the project yourself?

You can act as your own project manager, but it’s a big commitment. You’ll need to:

• Hire designers and trades
• Schedule deliveries
• Manage the budget and timeline
• Deal with building control and regulations

If the project is complex or includes structural changes, it’s usually safer to hire a main contractor. It might cost more upfront, but it can save you time and stress later.

Living on-site during building work

Living at home while the extension is built can slow things down. Most contractors recommend moving out during major work.

Make sure to plan this in your budget. In some contracts, your builder may agree to cover temporary accommodation costs if the work overruns – but only if it’s in writing.

Bigger isn’t always better

A huge extension might look impressive, but thoughtful design nearly always wins. Make sure the space suits your needs, flows with the rest of your home and adds value.

This is especially important if you’re staying within permitted development limits.

A good designer or architect will help you get the best out of your budget and space.

Choosing the right builder

Once you’ve got your plans and permissions sorted, it’s time to hire someone to do the work.

• Ask friends, neighbours or local Facebook groups for recommendations
• Look for builders with strong reviews and clear portfolios
• Always get a detailed quote – and ideally a fixed price
• Avoid “day rate” agreements unless you know exactly what’s involved

You might also find it helpful to read our guide: How to choose a good builder.

Heating and electrics

If your extension adds significant space, your current boiler or radiators might not be powerful enough. It’s a good time to check if your heating system needs upgrading.

You can also take the opportunity to improve sockets, switches and lighting layout in both the new and existing rooms.

How much does a house extension cost in 2025?

House extension costs have risen in recent years due to material and labour pressures, so it’s important to budget carefully.

Average costs in 2025 (per square metre, excluding VAT):

Example: A 30 m² single-storey kitchen extension could cost between £54,000 and £105,000, depending on quality, finishes and location.

London tends to cost 10–20% more than the UK average. Build costs also fluctuate based on property type, access, planning needs and finish level.

On top of that, budget for:

• Architect and design fees
• Structural engineer and building control
• Planning application (if needed)
• Surveys and party wall agreements
• Insurance
• Temporary accommodation (if moving out)

Want to keep costs down?

Here are a few ways to reduce your extension costs:

• Keep the design simple
• Use standard-size doors, windows and fittings
• Minimise changes to drains and utility services
• Get multiple quotes from reputable builders
• Finalise your plans before work begins – changes mid-project often cost more

Final thoughts

Planning a house extension takes time and thought, but it can transform how you live in your home. Whether you want a bigger kitchen, an extra bedroom or just more natural light, good planning is key.

Speak to professionals early on and take your time choosing the right team. The right design and build team will help you avoid mistakes, manage the budget and create a home that works for years to come.

Sources

• Homebuilding & Renovating – Extension Costs Guide
• Checkatrade – House Extension Costs
• Livingetc – How Much Does an Extension Cost?
• BuildPartner – Extension Costs in 2025
• Gov.uk – Party Wall Act Guidance