Your Extension Questions, Answered

Extending your home is a big decision, but also one of the most rewarding processes – you get a space tailored to you at the end. Whether you’re adding a kitchen or a full-blown double-storey extension, it’s important to understand what’s involved before you dive in.

Our renovation coach speaks to homeowners daily. Here we’ve answered all the most common questions asked about extensions, so you can move forward with clarity and confidence.

1. What does the roadmap for an extension look like?

Here’s a typical step-by-step breakdown of how a home extension unfolds:

1. Initial research and goal setting
   Think about what you want to achieve. More living space? Better light? Higher property value? Also consider your timeline and budget.
   
2. Feasibility and concept design
   Work with a designer or architect to check what’s possible based on your home and if there are any planning constraints.
   
3. Planning stage
   Depending on the project, you might need full planning permission, permitted development approval or a lawful development certificate.
   
4. Detailed designs and structural plans
   You’ll need to get technical drawings and structural calculations for building regulations and construction.
   
5. Tendering and quotes
   The tender process involves writing a clear scope of work and gathering your documents and drawings into a tender pack to send to potential builders for accurate quotes. Once you receive quotes and conduct site visits, you can make an informed decision about which building team to hire.
   
6. Pre-construction admin
   Obtain party wall agreements, building control notices and insurance, and prep the site.
   
7. Construction begins
   Your builder breaks ground and work is carried out in stages.
   
8. Final snagging and handover
   You’ll inspect the work, flag anything that needs fixing, and get final sign-off.

2. How do I know if my property is suitable for an extension?

Many London homes can be extended to some degree. Here are the key things to consider:

• Plot size – Is there space to extend?
  
• Access – Will builders and equipment be able to reach the site?
  
• Site conditions – Soil type, drainage and trees can all impact the project
  
• Structural limits – Older homes may need strengthening work done first
  
• Planning context – Some areas have tighter restrictions

3. How much should I be spending on my extension?

The cost of a rear extension and price of a side return extension in London varies hugely depending on the size and scope of the project and the materials and fittings you choose to use – the latter can limit or drive up the cost hugely. 

As a rough guide:

The average size of a rear extension in London is 45m² and you should expect to pay between £85,500 and £135,500 for this.

For ballpark total costs for a side return extension in London, you’re looking anywhere between £70,000-£250,000. Recent side return extension projects that we’ve worked on have cost on average £120,000–£150,000 for labour and building materials and £50,000 for client supply items, plus VAT.

Admin and professional fees need to be factored in as well:

• Architectural drawings: £2,000–£5,000+
  
• Planning fees: ~£200
  
• Structural engineering: £500–£2,000
  
• Party wall surveyors: £1,000–£3,000 (if needed)
  
• Building control fees: £500–£1,000
  
  The Page can provide you with costs for similar projects in terms of complexity, in your area, and in similar property types. We have up to date accurate information and market data on labour, build and material costs and how you can manage a budget in terms of where to save and where to spend as well as notify you of miscellaneous and usually hidden costs so you have no surprises. We will equip you with the knowledge to accurately and confidently budget for your project. 
  

4. What’s permitted development?

Permitted development is a set of government rules that allow homeowners to carry out certain extensions and alterations without applying for full planning permission. It applies to:

• Rear and side extensions
  
• Loft conversions
  
• Garage conversions
  
• Outbuildings
  

Permitted development rights can be restricted if:

• Your property is in a conservation area or national park
  
• It’s been previously extended beyond permitted development limits

Permitted development can only be used for houses – if you live in a flat or maisonette, you’ll need to get planning permission.

5. Who can tell me if my extension needs planning permission?

The best place to start is with your local council’s planning department. They’ll be well-versed in planning rules and regulations for your area and will be able to advise on the best next steps – they will also be the ones reviewing your application if you do need planning permission. If you’re using an architect, they’ll be able to advise you based on previous projects.  

Whether or not you need planning permission depends on:

• Your property’s planning history
  
• The type and size of extension
  
• Your location (e.g. if it’s in a conservation area)
  
• Local council policies

Some extensions fall under permitted development rights, which allow certain types of work without needing full planning approval. You can avoid planning permission if:

• Your extension is within specific size and height limits
  
• It doesn’t significantly alter the front of the property
  
• It’s not in a conservation area or on a listed building
  

Even under permitted development, you still need Building Regulations approval, and often a Party Wall Agreement if you’re near a boundary.

6. How far can I extend without getting planning permission?

You can avoid planning permission if your extension plans are of a certain size. These are the current permitted development rules in England:

• Detached houses: extend up to 4 metres at the rear
  
• Semi-detached or terraced: up to 3 metres

You may be able to go up to 6 or 8 metres with a Prior Approval application, which is simpler than full planning but still requires council review and neighbour consultation.

7. What value will an extension add to my home?

It varies hugely depending on the quality of the workmanship and the finish, but ballpark figures are:

• A rear kitchen-diner extension adds 10–15%
  
• A double-storey extension with an extra bedroom and bathroom can add 15–20%
  
• A well-done loft conversion adds 10–15%
  

The return depends on local property prices and how well the extension meets buyer demand.

8. What are the most common challenges with extension projects?

Some of the top issues homeowners face include:

• Underestimating costs (especially finishes and admin)
  
• Planning permission delays
  
• Neighbour disputes (usually from party wall or light issues)
  
• Supply chain delays for materials
  
• Choosing the wrong builder for the project
  
• Scope creep (adding extras mid-project)
  

With a clear plan and the right support, most of these challenges can be managed or avoided.

9. How do I budget for a home extension?

Start with a cost per square metre and calculate a base cost using your desired extension square footage. £3,225 is about average for building and labour costs in London, but the quality of materials used impacts this. You could be looking at:

• Basic: £2,000/m²
  
• Mid-range: £2,500–£3,500/m²
  
• High-end: £4,000+/m²
  

Then add:

• 10–15% for design and admin (planning, engineers, approvals)
  
• 10–20% contingency for surprises
  
• The cost of client supply items (e.g. kitchen cabinets, light fittings, carpet)
  
• VAT (often forgotten in early estimates)
  

10. How long does an extension take?

Timelines vary depending on complexity, builder availability and weather. Smaller projects can move quicker, but always allow buffer time. A typical extension project spans six to 12 months, broken into distinct phases. Design and planning usually take two to three months, obtaining permissions (if you need them) might require two to four months, construction typically runs for three to six months, with a final one to two months for finishing touches and addressing any final snags. 

Get matched to the right builder for your project with The Page

The Page is a free London-based renovation coaching and builder matching platform. We provide homeowners with a renovation roadmap: achieve your dream space with clarity and bespoke support throughout the process, transparency on budget and real market costs, and the expert workmanship of our Pagers. Pagers are the hand-picked, hands-on builders we work with. Highly vetted, skilled, and experienced, with tight-knit teams, their lean businesses equal top-quality work for a fair price.

Thomas is our in-house renovation coach. He intimately understands the renovation process and project-specific challenges – he’s helped over 1,000 Londoners renovate with confidence. Whether you’re transforming your kitchen into a host-with-the-most space or adding an extension to maximise family time, Thomas is here to signpost you at every stage of your project so you can make informed decisions. From budgeting and sourcing fittings, right through to the pinch-me moment when you’re raising a toast to your new space, he’s here to help. 

The result? Confidence in your investment and a home you love.