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GuideHow long does a house extension take?
A house extension has two stages: getting the design and approvals in place, then the build itself. Here is how long each stage usually takes in London, what happens on site stage by stage, and what can make it take longer.
The short answer
Most single-storey extensions take 12–20 weeks on site, depending on the type. The design and approvals stage comes before that.
- Rear extension: 12–16 weeks.
- Short side-return extension: 12–16 weeks.
- Long side-return extension: 14–18 weeks.
- Wraparound or L-shaped extension: 14–20 weeks.
Before the build
- Measured survey and drawings. We survey the house and draw the existing layout and the new design.
- Planning. Many single-storey rear extensions are permitted development, which can be confirmed with a lawful development certificate. Larger extensions, and most work in conservation areas, need full planning permission. Councils aim to decide householder applications within 8 weeks.
- Party wall notices. These must be served before work starts near a shared wall: 1 month's notice for a new wall on the boundary, and 2 months' for work to an existing party wall.
- Building regulations and structural design. Structural calculations and building regulations approval, so the build can start once everything is in place.
On site, in order
- Setting up the site and demolition.
- Foundations and drainage.
- Walls, steelwork and roof.
- Windows, bifold or sliding doors and roof lights.
- First fix: electrics and plumbing.
- Plastering.
- Second fix, kitchen and flooring.
- Snagging and handover.
What can make it take longer
- Conservation areas and Article 4 streets, where more of the design needs planning permission.
- Neighbours appointing their own party wall surveyor.
- Lead times for steels, glazing and other made-to-order items.
- Combining an extension with a loft conversion, which adds to the programme.
How we keep it on track
One team handles the drawings, planning and build, with a single project lead from start to finish. We agree the programme with you before work starts.
Questions homeowners ask
How long does a single-storey rear extension take to build?
Most single-storey rear extensions take 12–16 weeks on site. Side-return extensions take 12–18 weeks and wraparound or L-shaped extensions 14–20 weeks, depending on size and specification.
How long does planning permission take for an extension?
Councils aim to decide householder planning applications within 8 weeks of the application being validated. Many single-storey rear extensions are permitted development instead, which can be confirmed with a lawful development certificate.
Do I need a party wall agreement before my extension starts?
If the work involves a shared wall, a new wall on the boundary or digging foundations close to a neighbour's, you need to serve party wall notices before work starts: 1 month's notice for a new wall on the boundary and 2 months' for work to an existing party wall. We arrange this through our partner party wall surveyors.
Plan your extension
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