House Extension Timeline
A realistic house extension timeline is the single most useful thing you can build before you spend a penny on bricks or steel. Most homeowners underestimate how long an extension takes, and that mismatch between expectation and reality causes stress, budget overruns and strained relationships with builders. This article sets out how long each stage actually takes in the UK, from your first sketch through to building control sign off, along with the decisions and permissions that shape the schedule.
Having managed and specified extensions of every size, I can tell you the build itself is rarely the longest part. Design, planning and pre construction admin often eat up more calendar time than the physical work. Get those stages right and the rest follows more predictably.
How long does a house extension take from start to finish?
For a typical single storey rear extension of around 20 to 30 square metres, expect roughly 6 to 9 months from first design conversation to completion. The on site build alone is usually 10 to 16 weeks. A two storey extension adds structural complexity and often stretches the build to 16 to 24 weeks.
The gap between those two figures, the total timeline versus the build time, is where the design, permissions and procurement work sits. That preparatory phase commonly takes 3 to 5 months and it is where projects either gain momentum or grind to a halt.
Here is a broad breakdown of a standard single storey extension timeline.
| Stage | Typical duration | What happens |
|---|---|---|
| Design and concept | 3 to 6 weeks | Architect or designer produces initial drawings and options |
| Planning or permitted development | 6 to 10 weeks | Application, consultation and decision |
| Building regulations and structural design | 3 to 6 weeks | Detailed drawings, structural calculations, building control |
| Tendering and hiring a builder | 3 to 6 weeks | Quotes, references, contract, scheduling |
| On site build | 10 to 16 weeks | Groundworks, structure, first fix, second fix, finishes |
| Snagging and sign off | 1 to 3 weeks | Final inspections, completion certificate, defects |
Stage one: design and concept
Everything starts with a clear brief. Before you contact an architect, write down what you actually want the extension to do. A bigger kitchen, a downstairs bathroom, a home office, more natural light. Vague briefs produce vague drawings and endless revisions.
An architect or architectural technologist typically charges between 8 and 12 per cent of the build cost for a full service, or you can pay for drawings only at a lower fee. For a straightforward rear extension, expect to pay £1,500 to £4,000 for planning and building regulations drawings.
Allow 3 to 6 weeks for this stage, longer if you go through several rounds of revisions. Booking a designer can itself take a few weeks because good ones are busy. Start early.
Decisions made here ripple through the whole project. Roof style, glazing, whether you want bifold or sliding doors, underfloor heating, all of these affect cost and lead times later. Nail them down now rather than mid build.
Stage two: planning permission or permitted development
Not every extension needs full planning permission. Many single storey rear extensions fall under permitted development rights, which let you build without a full application provided you stay within set limits.
Under permitted development, a single storey rear extension on a detached house can extend up to 4 metres from the original rear wall, or 3 metres for a semi or terrace. Larger extensions up to 8 metres detached or 6 metres attached may be possible under the prior approval process. You can check the current rules on the government's Planning Permission guidance.
I always recommend applying for a Lawful Development Certificate even when you are confident the work is permitted. It costs around £120 and gives you documented proof, which matters enormously when you come to sell. Determination takes about 8 weeks.
If you need full planning permission, the local authority has a statutory 8 week determination period for householder applications. In practice, delays, extra information requests and validation issues can push this to 10 or 12 weeks. The application fee for a householder extension in England is currently £258.
Two storey extensions almost always need full planning permission and involve more scrutiny of overlooking, overshadowing and the impact on neighbours. For a detailed walk through of that process, read Planning a Two Storey Extension in the UK Timeline, Key Decisions and Common Hold Ups to Avoid.
Party Wall matters
If your extension involves building on or near a shared boundary, excavating close to a neighbour's foundations, or working on a party wall, the Party Wall etc. Act 1996 applies. You must serve notice on affected neighbours, usually 2 months before work starts for wall works and 1 month for excavation.
This is a frequent hidden delay. If a neighbour dissents, you may need a party wall surveyor and an agreed award, which adds £700 to £1,500 per surveyor and several weeks. Serve notices early.
Stage three: building regulations and structural design
Planning permission governs what your extension looks like. Building regulations govern whether it is safe and sound. These are separate approvals and you need both.
Building regulations cover structure, fire safety, insulation, ventilation, drainage and energy efficiency. Your options are a full plans application submitted to building control before work starts, or a building notice for simpler jobs. For most extensions I favour full plans because you get approval before anyone digs.
You will need a structural engineer to design any beams, steels or foundations. Engineer fees for an extension typically run £500 to £1,200. Their calculations feed into your building regulations drawings.
Building control can be handled by your local authority or an approved inspector. Fees usually total £500 to £1,000 depending on the project size. Allow 3 to 6 weeks to finalise drawings and get plans approved, though this can overlap with planning to save time.
Stage four: tendering and hiring your builder
Once you have approved drawings and a structural design, you can get accurate quotes. Vague drawings produce vague prices, which is why I never recommend hiring before this point.
Get at least three quotes from builders working to the same specification. If the numbers vary wildly, it usually means they have priced different scopes. Compare like for like.
Good builders are booked weeks or months ahead. Between quoting, checking references, agreeing a contract and waiting for a start slot, allow 3 to 6 weeks minimum. For extensions worth more than around £30,000, use a written contract such as the JCT Homeowner Contract to protect both parties.
A structured approach to organising quotes, contracts and scheduling helps here. The Home Renovation Timeline Template for Homeowners from First Quote to Final Sign Off is a useful framework for keeping every stage on track.
Stage five: the on site build phase of your house extension timeline
This is the visible part, and it follows a logical sequence. Understanding the order helps you spot when things drift.
- Setting out and groundworks (1 to 2 weeks). Marking out, digging foundations, building control inspects the trenches before concrete is poured.
- Foundations and oversite (1 to 2 weeks). Pouring footings, laying the concrete slab or beam and block floor.
- Superstructure (3 to 5 weeks). Brick and block walls, installing steel beams, forming door and window openings.
- Roof (2 to 3 weeks). Roof structure, covering, making the extension watertight.
- First fix (2 to 3 weeks). Windows and doors, electrics chased in, plumbing pipework, insulation.
- Plastering (1 to 2 weeks). Boarding and skimming walls and ceilings, then drying out.
- Second fix (2 to 3 weeks). Sockets, switches, radiators, sanitaryware, kitchen units.
- Finishes and decoration (1 to 2 weeks). Flooring, tiling, painting, final fittings.
Building control will inspect at key stages, typically foundations, damp proof course, drainage, insulation and completion. These inspections must be booked and can cause short pauses if not scheduled ahead.
Electrical and gas work
Any electrical work must comply with Part P of the building regulations and should be carried out by a registered electrician, ideally one certified by a scheme such as NICEIC. Any gas work, including moving a boiler or hob, must be done by a Gas Safe registered engineer. Never accept work from anyone who cannot show current registration.
Open plan layouts add complexity
Many extensions knock through into existing rooms to create open plan living. That sounds simple but it introduces structural work, careful sequencing and multiple trades working in the same space. Get the order wrong and you pay for rework.
Removing a load bearing wall to open up the space requires a steel beam and building control approval. Coordinating flooring levels, heating zones and lighting across old and new areas needs planning. For guidance on sequencing this well, read How to Plan an Open Plan Renovation Timeline Without Bottlenecks or Rework.
Cost breakdown for a typical single storey extension
Timeline and budget are linked. Delays cost money and cutting corners on budget usually causes delays. Here is a realistic cost picture for a 25 square metre single storey rear extension in 2026, excluding London where prices run higher.
| Item | Typical cost range |
|---|---|
| Architect or designer fees | £1,500 to £4,000 |
| Structural engineer | £500 to £1,200 |
| Planning and building control fees | £750 to £1,500 |
| Party wall surveyor (if needed) | £700 to £1,500 |
| Build cost (£2,000 to £3,000 per square metre) | £50,000 to £75,000 |
| Kitchen, bathroom or high end finishes | £8,000 to £25,000 |
| Contingency (10 to 15 per cent) | £6,000 to £11,000 |
Total budgets for a well finished single storey extension commonly land between £65,000 and £100,000. Two storey extensions cost more overall but less per square metre because you share foundations and roof across two floors.
Building a buffer into your timeline
Every experienced builder knows delays are not a possibility, they are a near certainty. Material lead times, weather, inspection scheduling and unexpected ground conditions all bite.
Add a realistic buffer rather than hoping for the best. I advise clients to add 15 to 20 per cent to the build timeline and hold a contingency fund of 10 to 15 per cent of the budget. For a deeper look at planning around delays and lead times, read How to Plan a Home Renovation Buffer Timeline for Delays Inspections and Material Lead Times.
Long lead items deserve special attention. Steel beams can take 2 to 4 weeks, made to measure aluminium bifold doors 6 to 10 weeks, and bespoke rooflights or brick matches even longer. Order these as soon as your design is fixed.
Common mistakes to avoid
Over years of seeing extensions succeed and stall, the same avoidable errors keep appearing.
- Hiring a builder before drawings are finalised. You cannot get accurate prices without a firm specification. This causes disputes and mid build cost rises.
- Ignoring the Party Wall Act. Skipping notices to save time can bring work to a legal standstill and sour relations with neighbours.
- Underestimating lead times. Ordering windows or steels late leaves your builder standing idle while you pay for their time.
- No contingency fund. Ground conditions, drains and existing structure often hide surprises. Without a buffer, one problem stops the whole job.
- Confusing planning with building regulations. They are separate. Having one does not mean you have the other.
- Vague or verbal agreements. Always use a written contract with a clear scope, payment schedule and start date.
- Changing your mind mid build. Variations are the biggest cause of both delays and budget blowouts. Decide finishes and layout before work starts.
- Not checking trade credentials. Always verify Gas Safe and electrical scheme registration before work begins.
A practical pre build checklist
Before your builder starts on site, tick off the following. If any item is missing, expect delays.
- Planning permission granted or Lawful Development Certificate secured
- Building regulations full plans approved
- Structural engineer's calculations complete
- Party wall notices served and awards agreed where needed
- Written contract signed with agreed payment schedule
- Long lead items ordered, including steels, windows and doors
- Contingency fund set aside and accessible
- Kitchen and bathroom specifications finalised
- Access, parking and skip or waste arrangements agreed
- Building control inspection stages understood and booked as required
Frequently asked questions
Do I need planning permission for a single storey extension?
Not always. Many single storey rear extensions fall under permitted development rights, which allow up to 4 metres of depth for a detached house or 3 metres for a semi or terrace, subject to height and boundary limits. Larger extensions may qualify under the prior approval process. However, permitted development rights do not apply in conservation areas, on listed buildings, or where they have been removed by an Article 4 direction. Even when work is permitted, I recommend applying for a Lawful Development Certificate so you have proof for future sale. If in doubt, contact your local planning authority before starting.
How long does the on site build take for an extension?
A single storey rear extension typically takes 10 to 16 weeks on site, assuming no major surprises and reasonable weather. A two storey extension usually takes 16 to 24 weeks because of the additional structural work, roof and internal alterations. These figures cover the physical build only. The full project, including design, permissions and hiring a builder, usually runs to 6 to 9 months for a single storey and 9 to 12 months for a two storey extension.
What is the difference between planning permission and building regulations?
Planning permission controls whether you can build and what it looks like, considering factors such as size, appearance and impact on neighbours. Building regulations control how you build, covering structural safety, insulation, fire safety, drainage and ventilation. They are two separate approvals from different parts of your local authority or, for building control, potentially an approved inspector. You generally need both. Having planning permission does not mean your design meets building regulations, and permitted development work still has to comply with building regulations.
How much should I budget for an extension in the UK?
Build costs for a single storey extension typically run £2,000 to £3,000 per square metre outside London, so a 25 square metre extension costs roughly £50,000 to £75,000 for the shell and fit out. Add professional fees, planning and building control charges, and a kitchen or bathroom, and total budgets commonly land between £65,000 and £100,000. Always hold a contingency of 10 to 15 per cent. London and the South East run 20 to 40 per cent higher. Higher end finishes, complex glazing and difficult ground conditions push costs up further.
Can I live in my home during an extension?
Usually yes, for a single storey rear extension, because the work is largely separate from your existing living space until the knock through stage. Expect dust, noise and restricted access, and plan for the period when the connection between old and new is opened up. For two storey extensions or projects involving significant internal reconfiguration, moving out for part of the build is often more practical. Discuss this with your builder early so the sequence can be planned to keep at least a kitchen and bathroom usable.
Final Thoughts
A successful extension is really an exercise in sequencing. The build is the visible part, but the design, permissions and procurement stages determine whether the whole thing runs smoothly. Give each stage the time it genuinely needs, build in a buffer, and lock your decisions down before work starts.
The homeowners who finish on time and on budget are the ones who prepared thoroughly, hired verified trades and refused to skip the paperwork. Treat your timeline as a living plan, review it weekly with your builder, and act quickly when something slips.
Ready to get started? post a job on BookaBuilderUK to receive free quotes from vetted local tradespeople who can turn your extension plans into reality.
