A single storey extension takes five to nine months from the first drawing to handover. The build itself runs 8 to 16 weeks. Design, approvals and lead times fill the rest, and most homeowners leave those stages out of their plans.
Ask a builder how long an extension takes and you’ll hear “about three months”. That figure covers the time on site. It leaves out the drawings, the paperwork and the wait for your bifold doors, which together can double the total.
Below is the full sequence for a typical single storey rear extension in England, with a realistic duration for each stage.
The Timeline at a Glance
| Stage | Typical time |
| Measured survey and design drawings | 3 to 5 weeks |
| Planning permission (if needed) | 8 weeks from validation |
| Prior approval (larger home extension) | Up to 42 days |
| Structural calculations and building regulations | 3 to 5 weeks |
| Party wall notices | 1 to 2 months, alongside approvals |
| Build on site | 8 to 16 weeks |
| Final inspection and snagging | 1 to 2 weeks |
Under permitted development, plan on five to six months end to end. A full planning application pushes that to seven to nine.
Stage 1: Survey and Design (3 to 5 weeks)
An architect or architectural technologist measures your house, then draws the existing layout and the proposed extension. Expect one or two rounds of changes before you sign off. Each round adds about a week, so decide early on the choices that drive the structure: the depth, the roof type and the width of the opening into the house.
Fix your kitchen layout now too. Your drains, windows and steel positions all follow the sink and the island, and moving them after the floor slab goes down costs money.
Stage 2: Planning Permission or Permitted Development (0 to 3 months)
This stage moves your timeline more than any other.
Many single storey rear extensions fall under permitted development, so you skip the planning application. You can extend 4 metres beyond the original rear wall of a detached house, or 3 metres on a semi or terrace, and the extension can’t exceed 4 metres in height.
Need more depth? The larger home extension scheme allows 8 metres on a detached house and 6 metres on other houses through prior approval. The council notifies your neighbours, who get 21 days to comment. You can start once the council approves or once 42 days pass without a decision.
Anything outside those limits needs a householder planning application. Councils aim to decide within eight weeks of validating it, and validation can take another week or two if the planners ask for more information.
Building under permitted development? Apply for a Lawful Development Certificate as soon as your drawings are final. It proves the extension was legal, and a buyer’s solicitor will ask for it when you sell.
Flats and maisonettes have no householder permitted development rights, and some newer estates had theirs removed by planning condition. Check your deeds and the estate’s original permission before you design around the limits.
Stage 3: Structural Design and Building Regulations (3 to 5 weeks)
Every extension needs building regulations approval, whether or not it needs planning. A structural engineer sizes the steel beam over the new opening and designs the foundations. Your designer then adds construction details to the drawings.
You can choose between two routes:
- Full plans application. Building control checks the drawings before work starts and must decide within five weeks, or two months if you agree to an extension.
- Building notice. You give two working days’ notice and building control inspects as the work goes up. Nobody checks the design before your builder digs.
For an extension with a steel beam and new foundations, go with full plans. You can submit them while your planning application sits with the council and save a month.
Stage 4: Party Wall Notices (1 to 2 months)
If your extension sits on or near a shared boundary, the Party Wall etc. Act 1996 will probably apply. Cutting a beam into a party wall needs two months’ notice. Building a new wall on the boundary line, or digging foundations within 3 metres of your neighbour’s house and deeper than theirs, needs one month.
If your neighbour doesn’t reply within 14 days, the Act treats the silence as a dispute and you’ll need a party wall surveyor. Serve your notices the week your drawings are fixed so the notice period runs alongside planning and doesn’t push back your start date.
Stage 5: Order Long Lead Items Early
Bifold doors, sliding doors, roof lanterns and fitted kitchens can take 4 to 10 weeks to arrive after you confirm sizes. Order them once building control approves the drawings. Your builder can’t plaster, screed or fit a kitchen in a room with no back wall.
Stage 6: The Build, Week by Week (8 to 16 weeks)
A typical 12 to 14 week build for a rear extension with a flat roof and a lantern runs like this.
Weeks 1 to 2: groundworks. Your builder sets up the site, digs the foundations and books a building control inspection of the trenches before any concrete goes in. Drains go in or get diverted here. If a public sewer runs under the footprint, you’ll need a build over agreement from your water company, and you want that signed before day one.
Weeks 3 to 5: walls and floor. The bricklayers take the walls up to roof height. The floor insulation and concrete slab go down.
Weeks 5 to 7: roof, doors and knock through. The roof goes on, then the windows and doors. Once the extension is weathertight, the builder fits the steel and opens up the back wall. Most families live at home through the whole build. Plan around the knock through week, as it brings the most dust and noise.
Weeks 7 to 10: first fix and plastering. Electricians and plumbers run cables and pipes, then the plasterer skims the walls and ceiling.
Weeks 10 to 14: screed, second fix and kitchen. Screed catches most people out. The common rule for a sand and cement screed is one day of drying per millimetre for the first 40mm, and slower after that, so a 50mm floor can need around two months before wood or LVT goes down. If your flooring can’t wait, ask your builder about a fast drying screed at the design stage.
Final week: sign off and snagging. Building control carries out a final inspection and issues a completion certificate. Keep it with your electrical certificate and house paperwork. You then walk the room with your builder, list any snags and agree a date to fix them before handover.
Common Causes of Delay
- Design changes after the drawings reach building control
- Clay soil near mature trees, which needs deeper foundations
- Drains found in the footprint once digging starts
- A neighbour who dissents from a party wall notice
- Doors or lanterns ordered after the shell goes up
Winter starts lose days to rain and frost during groundworks and roofing, so add a week or two if you break ground between November and February.
Keeping Your Extension on Schedule
Three habits protect your timeline. Get a quote that names the spec for every finish, so you aren’t choosing tiles while the plasterer waits. Ask for a written programme with week by week milestones before anyone digs. And pay in stages tied to finished work, so payment always follows progress.
MTR Developments, a Bromsgrove based builder working across Birmingham, gives every client a clear timeline before work starts and takes no upfront payment. Their guide to single storey extensions covers guide prices, typical build times and the planning rules in more detail.
Frequently Asked Questions
How long does a single storey extension take to build?
The build on site takes 8 to 16 weeks. From the first drawing to handover, allow five to six months under permitted development and seven to nine with a full planning application.
Can I start building before planning permission comes through?
No. If your extension needs planning permission, you must wait for approval. Under prior approval, you can start once the council approves or 42 days pass without a decision.
Does a single storey extension need building regulations approval?
Yes, every extension does. Building control inspects the foundations, drains and finished work, then issues a completion certificate you’ll need when you sell.
