How Long Does a House Extension Take in London? (2026 Guide)

One of the most common questions homeowners ask before starting a house extension project is how long it will take. The honest answer is that there are two timelines to understand: the build time on site and the total project timeline from your first consultation to completion. In London in 2026, the build time for a house extension is typically 10 to 18 weeks, but the full timeline including design, planning and approvals is usually 4 to 8 months.

LCCL Construction manages every stage of your house extension from design to completion

If you are wondering how long does a house extension take in London, the answer depends on the type of extension and how many pre-construction approvals are needed. LCCL Construction manages every stage from initial consultation through to building control sign-off and handover. This guide explains exactly how long each stage takes and what affects the overall timeline.

How Long Does a House Extension Take in London? The Full Timeline

Stage 1 — Initial consultation and survey: 1 to 2 weeks. LCCL Construction offers a free initial consultation where we visit your property, discuss your ideas and explain your options. A measured building survey follows to capture the existing layout accurately.

Stage 2 — Architectural design: 4 to 6 weeks. Our in-house architects produce full architectural drawings including floor plans, elevations and sections. We work closely with you to refine the design before submitting for planning or permitted development approval.

Stage 3 — Planning permission or permitted development: 4 to 12 weeks. Most single-storey rear extensions fall under permitted development and do not require a full planning application. If planning permission is required, a standard householder application takes 8 weeks in most London boroughs, though this can be longer in some areas.

Stage 4 — Building regulations and structural engineer: 2 to 4 weeks. Building regulations approval is always required regardless of whether planning permission is needed. LCCL Construction submits all building regulations applications and commissions structural engineer calculations simultaneously with the planning stage where possible to save time.

Stage 5 — Party wall agreements: 2 to 8 weeks. If your extension involves a shared wall or is close to the boundary, party wall agreements with your neighbours are required. LCCL Construction manages all party wall procedures. Most neighbours agree within 2 to 4 weeks, but the process can take longer if a party wall award is required.

Stage 6 — Construction: 10 to 18 weeks on site. The build phase starts once all approvals are in place. The exact duration depends on the type and size of extension.

How Long Does Each Type of House Extension Take to Build?

Rear extension (single storey): 10 to 12 weeks on site. The most common type of house extension in London, typically 3 to 6 metres deep. Full project timeline 4 to 6 months.

Side return extension: 10 to 14 weeks on site. Fills in the narrow side alley typical of Victorian and Edwardian terraces. Full project timeline 5 to 7 months.

Wraparound extension: 12 to 16 weeks on site. Combines a rear extension with a side return for an L-shaped addition. Full project timeline 6 to 9 months.

Double storey extension: 14 to 18 weeks on site. Adds space on both ground and first floor. Full project timeline 6 to 10 months.

What Can Delay a House Extension Project?

Planning delays: Planning applications in some London boroughs can take 10 to 12 weeks rather than the standard 8. Conservation areas and sensitive locations can slow the process further.

Neighbour disputes on party wall: If a neighbour objects to a party wall notice, a party wall surveyor must be appointed and a party wall award produced. This can add 4 to 8 weeks to the pre-construction phase.

Unexpected structural issues: Older London properties occasionally reveal unforeseen structural conditions once groundworks begin — additional steelwork or drainage diversions can add time to the build.

Design changes: Changes to the design after planning permission has been granted can require a planning amendment, adding further time. LCCL Construction works closely with clients at the design stage to agree the full scope before submission to avoid this.

Can I Stay in My House During a House Extension?

Yes — in most cases you can remain in your home throughout the build. House extensions are generally constructed from the outside and the opening into the existing house is created in the final stages of the build. LCCL Construction plans every project to minimise disruption to daily life.

How LCCL Construction Keeps Your Project on Track

As a design and build company based in Finchley, North London, LCCL Construction runs all pre-construction stages — design, planning, building regulations, structural engineering and party wall — simultaneously where possible. This reduces the overall project timeline significantly compared to managing each stage sequentially.

Every project has a dedicated project manager who keeps you updated throughout the build and coordinates all trades on site. To get a free consultation and detailed timeline for your house extension, call us on 020 8037 7332 or book online.