The short answer
From first drawing to finished room, a single-storey extension usually takes six to nine months. About half of that is design and approvals. Construction itself takes 10 to 16 weeks for a brick-built extension, or two to six weeks at your house for a factory-built one, with the factory work happening while the foundations go in.
On this page
Builders answer this question with the part they control: the weeks on site. That is the shortest stretch for most projects. The time before a spade goes in is longer, harder to predict and almost entirely paperwork.
The whole timeline at a glance
| Stage | Typical time | What sets it |
|---|---|---|
| Brief and drawings | 4 to 8 weeks | How quickly you decide |
| Planning decision, if needed | 8 weeks | The council’s target for householder applications |
| Building regulations plans | 5 to 8 weeks | Can overlap with planning |
| Party wall notices, if needed | 1 to 2 months | Your neighbour’s reply |
| Finding a builder and waiting for a start | 4 to 12 weeks | How busy good firms are |
| Construction, brick and block | 10 to 16 weeks | Weather, size, finishes |
| Construction, factory-built | 4 to 6 weeks in the factory, then 2 to 6 at the house | Access for delivery |
Several of these run side by side, so the total is less than the sum. Six months is a good result. Nine is common. A year is not unusual when planning is refused first time or a neighbour disputes a party wall notice.
Design: four to eight weeks
A measured survey of the house takes a day. First sketches follow in a week or two. What stretches this stage is choosing: how deep, how much glass, where the kitchen goes. Each round of changes costs a week.
You can shorten it by settling the big questions before the designer visits. Know your budget, which room it is and which wall it joins. Bring photographs of extensions you like.
Planning: nothing, six weeks or eight
Many single-storey rear extensions in England need no application at all, because they fall within permitted development. That saves two months. If you want written proof for a future buyer, a lawful development certificate takes up to eight weeks, but you do not have to wait for it before building.
A larger rear extension, between 3 and 6 metres deep on an attached house or 4 and 8 metres on a detached one, goes through prior approval. The council consults your neighbours and has 42 days to decide.
Everything else needs a householder planning application. Councils aim to decide those within eight weeks. Add two or three weeks beforehand to prepare the drawings and forms, and expect conditions that need sorting out afterwards. Scotland runs its own system with different limits and timescales.
Building regulations: five weeks, overlapping
Every extension needs building regulations approval. With a full plans application, the council has five weeks to decide, or up to two months if you agree to an extension. A private building control approver may be quicker.
There is no need to wait for planning before you apply, and many people submit both together. In Scotland the equivalent is a building warrant, and work must not start until it is granted.
Party walls and sewers
If you are building on or near a boundary shared with a neighbour in England or Wales, you may have to serve notice under the Party Wall Act: one month ahead for foundations dug close to their building, two months for work to a shared wall. If the neighbour agrees in writing, you can carry on. If they do not, surveyors are appointed and a few more weeks pass.
A public sewer under or beside the new room needs the water company’s consent too. Allow three or four weeks.
Construction the traditional way: 10 to 16 weeks
A brick and block extension is built in sequence, and most steps wait for the one before. Foundations and drains take two or three weeks. Walls and roof take four to six. Then the opening into the house is formed, followed by first-fix plumbing and electrics, plastering, drying, second fix, flooring and decoration.
Rain stops bricklaying and frost stops concrete. Wet plaster cannot be painted. A winter start can add a month. Throughout, the back of the house is a building site, and the kitchen is often out of action for much of it.
Construction in a factory: two to six weeks at the house
Factory-built rooms change the order. The foundations are dug at your home while the room is made indoors, so two jobs that normally follow each other happen together. After four to six weeks the room arrives by lorry, is craned onto the base in a day, and is weathertight that evening.
What remains is the opening into the house and the connections, which take two to six weeks. Because the existing wall stays in place until the new room is sealed, the house is open to dust for far less time. That is the main reason people choose modular house extensions: not the total, which is shorter by a couple of months at most, but the number of weeks the family lives with builders.
The method brings one extra item to plan for. If the crane needs the road closed, the council’s notice period for that can run to many weeks, so it has to be applied for as soon as the delivery date is known. Our page on delivery and cranes explains what is involved.
How to make yours quicker
- Check whether you need planning permission before you design anything. Drawing within the permitted development limits can save two months.
- Submit building regulations plans at the same time as planning.
- Speak to neighbours before the formal notices arrive. People who have been told in person rarely dispute.
- Order long-lead items early. Large sliding doors can take 10 to 12 weeks from some suppliers.
- Choose finishes before work starts. Late decisions about tiles and sockets are the most common cause of a stalled last fortnight.
For a sense of what each route costs, try the cost calculator. When you are ready, get a quote and ask the firm for a programme with dates on it, not a number of weeks.