The Stages of Building a House Extension: A London Timeline

Before work starts

1. Survey, design and drawings

A measured survey of the house, then design drawings. Allow roughly 4 to 8 weeks for a single-storey extension and 6 to 10 weeks for a double-storey, in our experience. The design decides almost everything that follows: whether you need planning permission, whether you are near a sewer, and what the structural engineer has to design.

2. Planning permission or a lawful development certificate

If the extension needs planning permission, councils are expected to decide householder applications within 8 weeks of validation, the day the application is accepted as complete, not the day it is sent. If it is permitted development, a lawful development certificate proves it, also with an 8-week target. In the year to June 2026, 93% of householder decisions in England were made in time. Our planning permission guide covers which route you need.

3. Party wall notices

Under the Party Wall etc. Act 1996, work on a shared wall needs at least two months' notice, and building on the boundary line or excavating near a neighbour's foundations needs one month. Neighbours have 14 days to reply in writing; if they do not consent, a dispute is treated as having arisen and one agreed surveyor, or one each, prepares an award. A neighbour can agree in writing to an earlier start. Our party wall cost guide explains the costs.

4. Building over or near a sewer

If the extension will be within 3 metres of a public sewer or 1 metre of a public lateral drain, Thames Water must agree to it before you build. Simple cases can be quick; complex ones can take months, so check this early. Our build-over agreement guide explains the process.

5. Structural design and building regulations

A structural engineer designs the foundations, steels and any underpinning. Building regulations approval comes from the council or a private registered building control approver, by one of two routes: full plans, where the drawings are checked before work and a decision usually follows within five weeks, or a building notice, where work can start two days after notice is given but without formal approval of the drawings. On larger extensions we recommend full plans.

6. Contract, programme and the CDM regulations

A written contract sets the price, programme and payments; for a homeowner, the JCT Building Contract for a Home Owner or Occupier is one common form, and we build on JCT contracts. Under the Construction (Design and Management) Regulations 2015, a homeowner's client duties pass automatically to the contractor, or to the principal contractor where there is more than one, unless a designer agrees in writing to take them on.

The build, week by week

These are typical stages and weeks on site for a single-storey rear extension, from our own projects. Larger, double-storey and wrap-around extensions take longer.

Weeks on site	Stage	What happens
Before week 1	Set-up	Hoarding, welfare, protection to the house and garden, skip or grab lorry access, any pavement licences
1–3	Demolition and groundworks	Old structures removed, foundations dug and inspected, concrete poured, new drainage laid and tested
3–7	Structure and walls	Damp-proof course and floor slab, walls built, steel beams installed to the engineer's design
6–9	Roof and weathertight	Roof structure, insulation and covering, rooflights, windows and doors
8–11	First fix	Electrics, plumbing, heating and underfloor heating run before walls and floors are closed in
10–14	Plastering and second fix	Plaster and screed, then sockets, switches, radiators, sanitaryware and the kitchen
13–16	Finishing	Flooring, decoration, external making good, snagging walk-round and handover

Plaster and screed must be dry before floors and decoration go on. Drying times depend on thickness, temperature and ventilation, so a moisture reading decides, not the calendar.

Breaking through into the house

Most families stay at home during an extension. The new part is built against the existing back wall, which is usually opened up near the end, once the extension is weathertight and the steel is in. The break-through itself typically takes one to three days, with temporary screens and dust control, and that is the most disruptive part of the job.

Building control inspections

Since 1 October 2023 the building regulations in England no longer list fixed inspection notices for each stage. Your building control body tells you which stages it will inspect and when it must be notified. Typical stages are the foundation excavation, the oversite and damp-proof course, drainage before it is covered, insulation and the structure before plastering, and completion. The statutory notices that remain include two days' notice before starting work and notice within five days of completion.

Completion certificate and electrical certificates

After the final inspection, building control issues a completion certificate, usually within eight weeks on the full plans route. Keep it: solicitors ask for it when you sell or remortgage, and missing paperwork can delay both. Electrical work is certified separately, by an electrician registered with a competent person scheme, who notifies building control for you. A completion certificate confirms the work met the regulations when inspected; it is not a guarantee of workmanship.

Paying for an extension

There is no legal cap on deposits. Which? advises a reasonable deposit, mainly where materials or bespoke items have to be ordered, payments tied to completed stages of work, and holding the final payment until you are satisfied. Stage payments against work done, with a small retention held for the defects period, is common practice, and a written contract should set this out before work starts.

Frequently Asked Questions

Sources

• Planning Portal: householder planning consent
• GOV.UK: planning applications in England, April to June 2026
• GOV.UK: the Party Wall etc. Act 1996 explanatory booklet
• Thames Water: developing over or near sewers
• GOV.UK: building regulations approval
• legislation.gov.uk: Building Regulations 2010, regulation 16
• HSE: CDM 2015 for domestic clients
• Which? Trusted Traders: how to work with your trader
• JCT: Building Contract for a Home Owner or Occupier

This guide is general information, checked against the sources above on 3 November 2026. Every house is different, so take advice on your own property before starting work.