How to Build a Home Extension Step by Step (2026 UK)
Once the drawings are approved and the builder is booked, an extension follows a fixed on-site sequence: set out, dig the foundations, build the walls, get a weathertight roof on, then first fix, plaster, knock through into the house, second fix and sign-off. Most single-storey builds run 8–12 working weeks on site. This guide walks you through every stage in order, how long each takes, when building control inspects, and what to check before you release each payment. For prices, see our house extension cost guide; for the design and permissions stage, read how to plan a home extension.
Follow the build in order
Building an extension is not one job but a chain of trades that must happen in the right order. Skip ahead and you pay for it later — plaster before first fix, or decorate before the knock-through, and work gets ripped out. Use the jump links below to move around, but on site the sequence runs top to bottom.
Want a quick estimate before you commit? Try the extension cost calculator or the cost-by-size tool, then come back here for the build itself.
Single-storey extension build timeline — 2026
Typical on-site duration for a straightforward single-storey rear or side extension. Two-storey and complex builds run longer; weather and foundation surprises are the usual delays.
Indicative on-site weeks for a single-storey extension · BestBuilders 2026 project data across 519 UK towns.
The 10 stages of building an extension
This is the order a competent builder works in. Each stage has a clear finish point — a good moment to inspect and to release the matching stage payment.
1. Set out & site setup
The builder marks the exact footprint from the approved drawings, confirms boundaries, and sets up access, welfare and a skip or waste plan. Existing drains and services are located now, not discovered later.
2. Foundations
Trenches are dug to firm subsoil. Building control inspects the open trench before any concrete is poured — depth depends on ground, trees and drains, so this is where surprises (and extra cost) surface.
3. Up to damp-proof course
Foundation blockwork is built to DPC level, the damp-proof membrane goes in, and the ground floor is laid — usually insulated concrete or beam-and-block. The footprint is now clearly readable on the ground.
4. Walls to roof level
Cavity walls go up with wall ties and cavity insulation. Lintels and any structural steel beams are fitted over window, door and opening positions. This is the fastest visible progress of the whole build.
5. Roof & weathertight
The flat or pitched roof structure is built and covered (EPDM, felt, GRP or tiles), then windows, doors and rooflights are fitted. Once the shell is weathertight, internal trades can start regardless of weather.
6. First fix
Electricians and plumbers run cables, water and waste before walls are closed up; carpenters build any stud walls and fix noggins where heavy fittings will hang. Everything is photographed before it disappears behind plaster.
7. Plaster & screed
Walls and ceilings are plastered and any floor screed is laid, then left to dry. Rushing decoration onto damp plaster is a common cause of cracking and staining — drying time is not wasted time.
8. Knock-through into the house
The structural opening between old and new is formed on its steel beam and made good. This is the dustiest, most disruptive day or two — it is left late so the new space is watertight and usable first.
9. Second fix & finishing
Sockets, switches, radiators, kitchen or bathroom units, internal doors, skirting and architrave go in, then decoration and flooring. The room starts to look finished, but this stage carries the most snags.
10. Snag & sign off
You and the builder walk the job and list every defect to put right. The final building control inspection follows, and you receive the completion certificate — keep it safe, you will need it when you sell.
How long each stage takes
Durations below are typical working weeks for a single-storey extension with no major complications. Two-storey work roughly adds 40–60%. See the full extension timeline guide for what pushes these out.
| Stage | Typical duration | Ends when… |
|---|---|---|
| Set out & groundworks | 1–2 weeks | Trench passed and slab down |
| Up to DPC & floor | ~1 week | DPC level reached, floor laid |
| Walls (superstructure) | 2–3 weeks | Wall plate on, ready for roof |
| Roof & weathertight | 1–2 weeks | Covered, windows and doors in |
| First fix & plaster | ~2 weeks | Plastered and drying |
| Knock-through & second fix | ~2 weeks | Opening formed, fittings in |
| Decoration, snag & sign-off | 1–2 weeks | Completion certificate issued |
Total on site for a straightforward single-storey build: roughly 8–12 weeks, plus the design and approvals stage beforehand.
Building regulations & inspections during the build
Almost every extension needs Building Regulations approval even when it is permitted development and needs no planning permission. The two are separate approvals — confirm both before groundworks start. Building control (your local authority or an approved inspector) signs the work off at set stages:
- Foundation excavation — the open trench is checked for depth and bearing before concrete.
- Damp-proof course & drainage — DPC/DPM continuity and any new drain runs.
- Oversite & insulation — floor build-up and insulation before it is covered.
- Structural elements — beams and lintels, especially the knock-through, against the engineer’s design.
- Completion — the final inspection that releases your completion certificate.
Your builder normally books these, but the certificate is issued to you as the homeowner — make sure you actually receive it. On planning, read do I need planning permission for an extension before you commit to a start date.
What to check at each stage
You do not need to be a builder to protect your project. These are the checks a homeowner can make before releasing each stage payment:
- Before foundations: a written, stage-based programme and payment schedule — never a large deposit up front.
- At DPC: the footprint matches the drawings; drains and DPC are as approved.
- At weathertight: windows and rooflights are the specified units, and the roof is genuinely watertight before internal work starts.
- Before plaster: photograph all first-fix cables and pipes, and confirm noggins are in where rails, TVs or radiators will hang.
- At second fix: test every socket, switch, tap and radiator; check units and doors line up.
- At completion: a written snagging list with dates, plus the building control completion certificate and any electrical and gas certificates.
We recommend you ask each builder for proof of their own insurance and any trade-body membership, and check it yourself — for example, ask to see public liability cover and confirm any scheme on its register. BestBuilders vets the builders in our network, but you should always verify credentials directly before signing.
Red flags during the build
- Concrete poured into a foundation trench that building control has not inspected.
- Pressure for a large up-front payment, or payments running ahead of completed work.
- Internal trades starting before the shell is genuinely weathertight.
- Plastering straight onto damp walls, or decorating over plaster that has not dried.
- The knock-through beam fitted without the engineer’s design or a building control check.
- No completion certificate offered at the end, or reluctance to provide electrical and gas paperwork.
Common Questions
Related Extension Guides
Plan the project, price it up, and check the rules before the build starts.