How Long Does a House Extension Take in London? Real Project Timelines

House extensions · Planning

What a London house extension takes, stage by stage: which steps run side by side, real AURA project timelines, and how to keep yours on track.

Last updated 13 October 2026 · AURA Architecture & Interiors, RIBA Chartered practice

The timings here are approximate and meant to give you an idea, not a programme. Every house and brief is different, so yours will depend on your project.

How long does a house extension take in London?

On our recent extensions, design and build together took about 10 to 12 months, as recorded on the project pages. Whitestile Road took 10 months, Bronson Road and Cromwell Road 11, and Maple Lodge 12. The build itself is about 4 to 6 months, and the stages before it overlap more than most people expect.

That range comes from four AURA projects where the project page or the job file records both design and build. Where a page gives design months and build months, the figure here is the two added together. Planning and tender sit within or alongside the design period, depending on the job.

The table shows each stage, what it takes on our projects or by law, and what can run at the same time.

Stage	What happens	Typical length on our projects (approx.)	Runs alongside	Your decisions
1. Brief and feasibility	First meeting, measured survey, what the house can take	Set by how settled the brief is	Ordering the measured survey	What you need the space for, your budget, when you want it finished
2. Concept and planning design	Options tested, then the chosen scheme drawn up for planning or a permitted development route	Set by how many options you test and how quickly one is agreed	The structural engineer’s first view; a check of drainage records	Layout, size, roof form and glazing
3. Planning, or the permitted development route	The council decides the application, or confirms the permitted development route	The official period is 8 weeks for a householder application, and 42 days for prior approval on a larger home extension	Party wall notices; technical design	Whether a permitted development route suits your house
4. Technical design and building regulations	Construction drawings, structural calculations, specification	On Maple Lodge, approx. 3 months from planning approval to the start on site, covering stages 4 to 6	Tender; Thames Water application	Windows, kitchen, finishes, heating
5. Party wall and Thames Water	Notices to neighbours; a build-over agreement where the extension is close to a public sewer	The official notice periods are at least 1 month or 2 months before work, depending on the work	Planning and technical design	Serving notices as soon as the design is fixed
6. Tender and contract (JCT)	Builders price the same drawings; the contract fixes the price and the programme	Part of the approx. 3 months on Maple Lodge (stage 4)	Building regulations; ordering long-lead items	Which builder, and the start date
7. Build	Groundworks through to finishes	Approx. 4 to 6 months on our single-storey rear extensions	The kitchen and joinery being made off site	Quick answers to site questions
8. Completion and sign-off	Snagging, final inspection, building control completion certificate	Set by the snag list and the final inspection	Moving back into the rooms	Checking the snag list; keeping every certificate

What are the stages, and which ones run at the same time?

Design comes first and the build comes last, but most of the steps in between can overlap. Party wall notices, a Thames Water application, structural design, building regulations and tender can all run while planning is decided or straight after it. That overlap is why a programme is shorter than its stages added up.

The design stage sets the pace for everything after it. A settled brief, an accurate measured survey and an engineer’s view at concept stage mean fewer changes later, when changes take longer. Each step is set out in our architectural design process.

For a householder application, the official period the council has to decide is 8 weeks, set out in the government’s guidance on determining a planning application. What affects that wait is covered in our guide to how long planning permission takes.

While the council decides, the technical design can start, the party wall surveyor can be appointed and the drainage records checked. Building control, which checks the construction itself, works from the technical drawings that builders price.

On our Maple Lodge project in Hertfordshire, planning was approved in February 2024 and work started on site in May 2024. Those three months covered the building regulations package, the tender and booking the builder, all running together.

Working back from the date you want it finished

Count back from the finish date. On our projects the build is about 4 to 6 months, so an extension you want finished by the end of September needs to start on site around April or May. Approvals, the technical design and tender need to be done by then, which puts the first design meeting in the autumn before.

Maple Lodge ran almost exactly to that pattern: first contact in October 2023, complete in October 2024.

Real AURA projects: how long each one took

On the rear extensions below, design and build together took about 10 to 12 months as the project pages record them, with builds of 4 to 6 months. Larger projects took longer for clear reasons: a whole house over split levels with a basement, or a Grade II listed house. Scope, more than planning, set the length of each programme.

Each project shows what its record holds, in months. Locations are as the project pages give them, and there are no client names. Most pages record design and build only, so planning and tender aren’t recorded separately: they sit within or alongside the design months, depending on the job.

Project	Location	Scope	Design	Build	Design and build (approx.)
Whitestile Road	Brentford, TW8	Rear kitchen extension to a period house	4 months	6 months (Apr to Sep 2021)	10 months
Bronson Road	Wimbledon	Rear extension of 3 to 4m plus a dormer loft	5 months	6 months (Oct 2021 to Mar 2022)	11 months
Cromwell Road	Wimbledon, SW19	4m full-width rear extension plus whole-house refurbishment	5 months	6 months (Apr to Oct 2018)	11 months
Maple Lodge	Hertfordshire (outside London)	Rear extension over 5m with a sedum roof	Approx. 4 months to approval, then approx. 3 months to the site start	5 months (May to Oct 2024)	12 months
Trinity Road	Wandsworth	Whole house over split levels, basement extended (a developer’s project)	5 months	18 months (from Q1 2022)	23 months

What shapes your timeline?

Five things set the length of an extension programme: how much you’re building, the consent route, the house itself, party walls, and sewers near the extension. On our projects, scope makes the biggest difference. Planning, party wall and Thames Water are mostly about starting early enough.

Scope

A rear extension alone builds in about 4 to 6 months on our projects. Bronson Road, with a dormer loft, and Cromwell Road, with a whole-house refurbishment, were both built in 6 months, within that range. Trinity Road, a whole house over split levels with the basement extended, had an 18-month build.

The consent route

Some extensions don’t need a full planning application. Under permitted development, a single-storey rear extension can go up to 6m on a semi-detached or terraced house, or 8m on a detached house, through prior approval, where the council has 42 days to decide (see the Planning Portal’s guidance on extensions). Fifth Cross Road, above, was decided in 38 days. Whether your house qualifies is covered in our guide to planning permission versus permitted development.

The house and its setting

Conservation areas, listed buildings and the existing structure all add design time. Works to a listed house need listed building consent as well as any planning permission, which is part of why Theobalds Manor and Hill Farm ran to 18 months. Older houses often need opening up before the engineer can finish the design.

Party walls

If your extension is on or near a shared wall, the Party Wall etc. Act 1996 sets notice periods. Building on the line of junction, or excavating near a neighbour’s foundations, needs at least one month’s notice; work to the party wall itself needs at least two months (Party Wall etc. Act 1996, section 3). A neighbour has 14 days to respond. Serve party wall notices once the design is fixed, so the clock runs while planning is decided.

Sewers and Thames Water

Building within 3 metres of a public sewer, or within 1 metre of a public lateral drain, usually needs a build-over agreement from Thames Water before work starts. Check the drainage records at concept stage and apply alongside planning, so it isn’t the last thing standing between you and the start date.

How can you keep an extension on track?

Start the slow paperwork early, choose the quickest consent route that suits the design, and make the big decisions before tender. Party wall notices and a Thames Water build-over application can run while planning is decided. A fixed specification lets builders price accurately, and a contract with a programme gives everyone a date to work to.

• Start party wall and Thames Water early. Serve notices once the design is fixed, and check the drainage records at concept stage, so neither waits until after planning approval.
• Use permitted development where it fits. If the design you want fits permitted development or prior approval, the decision period is shorter. Bending the design to fit a route rarely pays off.
• Get a measured survey and structural input early. Drawings built on accurate dimensions, with an engineer’s view from the start, need fewer changes later.
• Decide the spec before tender. Windows, kitchen, flooring and heating chosen early are priced properly and ordered in time. Long-lead items such as glazing, steel and the kitchen can be ordered as soon as the drawings are fixed.
• Tender while you wait. Builders can price the technical drawings while the last approvals come in, so a builder is ready to start when they do.
• Put the programme in the contract. A JCT building contract records the start date and the completion date, so progress is measured against something fixed.

Should you add a loft or refurbish at the same time?

If you’ll want a loft or a refurbishment within a few years, doing it with the extension usually makes sense. It takes longer to design and can add build time, but you live through one project, with one set of approvals and one builder. Phasing suits a budget that comes in stages, or a household that has to stay in.

Bronson Road built a rear extension and a dormer loft together, including the loft, in 6 months. Cromwell Road paired a full-width extension with a whole-house refurbishment, also built in 6. On both, the extra work shared one programme and one builder.

When we’d recommend doing it all at once

• the wiring, plumbing or structure will be opened up across the house anyway
• you can live somewhere else for the build
• the budget for the whole scope is in place
• the second phase would disturb the rooms the first phase finished

When we’d recommend phasing

• you need to live in the house during the work
• the budget comes in instalments, or the second phase is a few years away
• one part of the house is in good order and can be sealed off

If you phase, do the structural, dusty work first, and design phase two before phase one is built, so nothing finished gets taken apart later. Check whether a later phase needs its own consent, and when the first permission runs out.

What can go wrong, and how to avoid it?

Most programme slips come from four places: what’s under the ground, design changes after the builder starts, materials with long lead times, and wet weather before the roof is on. Each has a straightforward fix, and most of the fixes happen before anyone is on site.

What can go wrong	How to avoid it
Drains or ground conditions found when digging	Check the drainage records and survey the drains before the design is fixed. Keep time and money in reserve
Design changes once the build has started	Settle the design and specification before tender. Changes on paper are quicker and cheaper
Long-lead items arriving late	Order glazing, steel and the kitchen as soon as the drawings are fixed
Wet weather before the shell is up	Plan the start so the structure and roof go up in drier months where possible, and get the building weathertight early

On our Chestnut Grove project in Balham, a drain with an earlier extension built over it, and foundations under the party wall that needed correcting, were found on site. Putting them right took about two weeks out of a 4-month build.

Small finds like this are normal on old London houses. A survey of the drains before design, a contingency in the budget and a programme with a little slack keep them small.

Can you live in the house during an extension?

Often, for a rear extension, as long as you can manage without the kitchen for part of the build. It gets harder when the work reaches every floor or goes below ground. Decide early, because whether the house is empty changes how the builder can sequence the work.

Most rear extensions open into the kitchen, so expect a spell without one. A temporary kitchen in another room, with a sink, a hob and the fridge, makes staying in workable. Dust and noise still travel through a house, however well it’s protected.

A whole-house refurbishment or a basement is different. Services go off, floors come up and the stair may be out of use, so most people move out for at least part of it.

Living somewhere else is an option even for a smaller project. The Bronson Road clients lived abroad during the build, and we project managed the work and acted as contract administrator.

Frequently asked questions

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