The short answer
A single-storey brick and block extension takes about 10 to 16 weeks on site, and a two-storey one four to six months. Factory-made extensions are quicker, at two to eight weeks. Before any of that, allow three to five months for design, planning, building regulations and party wall notices.
On this page
The whole timeline at a glance
Most people ask about the building work and are caught out by everything before it. Here is the full run for a typical single-storey rear extension in England.
| Stage | Typical time |
|---|---|
| Survey, design and drawings | 4 to 8 weeks |
| Planning decision, if needed | 8 weeks |
| Building regulations plan check | Up to 5 weeks |
| Party wall notices, if needed | 1 to 2 months |
| Finding and booking a builder | 4 to 12 weeks |
| Building work on site | 10 to 16 weeks |
Several of these overlap, so the total is less than the sum. A realistic span from first meeting to moving furniture in is seven to twelve months.
Before the build
Design
A designer measures the house, draws options and, once you choose one, prepares planning drawings. A structural engineer then sizes the foundations and beams. Quick decisions from you make this stage four weeks. Second thoughts make it twelve.
Planning
Many rear extensions are permitted development and skip this stage. If you do need permission, councils aim to decide householder applications in eight weeks from the day the application is accepted as complete. The prior approval route for larger rear extensions takes up to 42 days.
In a conservation area, or with a listed building, expect longer and expect requests for changes.
Building regulations
With a full plans application, building control checks the drawings before work starts. The law gives a council five weeks, or two months if you agree to an extension. You can begin once the application is in, at your own risk.
Scotland works differently. You must hold a building warrant before starting, and how long that takes depends on the council and the quality of the application.
Party wall notices
If the work affects a shared wall, or you dig near a neighbour’s building, the Party Wall Act in England and Wales requires written notice: two months ahead for work on a party wall, one month for excavations. A neighbour who consents in writing lets you start sooner. One who does not triggers the appointment of surveyors, which adds weeks.
Booking the builder
Good builders are booked months ahead. Ask for prices when the planning drawings are done, not when approval lands, and you can cut the waiting.
On site: brick and block, week by week
Weeks 1 to 3: groundworks. Site set-up, digging, inspection of the trenches, concrete, blockwork below ground, drains and the floor slab.
Weeks 4 to 7: walls and roof. Blockwork and brickwork up to roof level, then the roof structure and covering. Rain and frost cause the most delay here.
Weeks 7 to 9: weathertight. Windows and doors go in. The opening into the house is cut and the beam fitted, usually behind a temporary dust screen.
Weeks 9 to 13: inside. First-fix electrics and plumbing, insulation, plasterboard and plaster. Plaster needs a week or more to dry before paint.
Weeks 13 to 16: finishing. Second fix, flooring, kitchen or bathroom fitting, decorating, making good outside and the final inspection.
A two-storey extension follows the same order with scaffolding up for longer. Four to six months is normal.
On site: factory-made extensions
A prefab extension moves much of that work into a factory, which shortens the spell at your house.
A panel-built room is typically on site for four to eight weeks. The foundations take the same two or three weeks, but the walls and roof then stand in days, already insulated, and the inside trades follow straight on.
A finished module is on site for two to six weeks. Most of that is groundwork before the lift and connection work after it.
A two-storey panel build usually takes eight to twelve weeks. The time before the build is no shorter, and factory lead time takes the place of waiting for a bricklayer.
What causes delays
Five things account for most overruns.
- Changes of mind. Moving a door after the wall is built costs days. Decide on paper.
- The ground. Soft spots, old drains and buried rubble only show once the digger starts.
- Glazing. Bifold and sliding doors can take many weeks to arrive. Order them early.
- Weather. Mostly a risk for brick and block between November and March.
- Waiting for inspections and utilities. Building control visits are quick to book. Moving a gas or electricity meter is not.
When is the best time to start?
Start the design in autumn and you can be through approvals by early spring, with the ground drying out and long days ahead. Start on site in October with brick and block and you will be laying bricks in the worst of the weather. Factory-made shells are far less sensitive to the season, which is one reason they suit winter starts.
How to shorten the wait
Stay within permitted development if the design allows. Appoint the engineer at the same time as the designer. Serve party wall notices the day the drawings are settled. Get builders pricing before approvals arrive. And choose a build method that fits your access, since the quickest method on paper is no use if it cannot reach your garden.
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