London Home Extension: Full Project Timeline from Idea to Completion 2025

The design development phase begins with the architect's first site visit and initial client brief and ends with a set of planning drawings ready for submission. For a typical London rear extension project, the design development phase takes 6-10 weeks. Week 1-2: brief and feasibility: the initial meeting with the architect covers the client's brief (what spaces are needed, how the extension will be used, approximate budget, planning constraints), a site visit to measure the existing property, and a feasibility discussion about what is achievable within the planning constraints, the budget, and the site. The architect also carries out initial desktop research: checking the planning history of the property, checking whether the property is in a conservation area or subject to an Article 4 Direction, checking the property's listing status, and checking the Local Plan policies that apply to the site. Weeks 2-6: concept design and planning drawings: the architect produces concept design options — typically 2-3 layout options — for the client to review and select. Once the preferred concept is selected, the architect develops it into a set of planning drawings (existing and proposed floor plans, existing and proposed elevations, and a site plan). This typically takes 4-6 weeks from the initial brief, depending on the complexity of the project and the client's decision-making speed. Weeks 6-8: design refinement and planning application preparation: the architect refines the planning drawings based on client review, prepares the supporting documents for the planning application (Design and Access Statement, Heritage Statement if required, Pre-Application advice response if applicable), and submits the application to the LPA. At the end of Stage 1, the planning application has been submitted. Note: this 6-10 week design stage assumes a clear brief from the client and reasonably straightforward planning constraints. A more complex project (conservation area, listed building, contentious planning history) may take significantly longer.

The statutory determination period for a householder planning application in England is 8 weeks (56 calendar days) from the date of a valid application. In practice, London LPAs frequently exceed this statutory period: in the busiest inner-London boroughs (Hackney, Islington, Lambeth, Lewisham, Southwark, Wandsworth, Camden), determination times of 10-16 weeks are common. Pre-application advice and its effect on the timeline: if the project has involved a pre-application advice consultation with the LPA before the formal application is submitted, the planning officer's familiarity with the proposal and the applicant's response to any pre-application concerns means the formal application is more likely to be determined quickly (and approved without significant revision). Projects without pre-application advice are more likely to receive planning officer queries, neighbour objections, and requests for additional information — all of which extend the determination timeline. Third-party objections and committee determination: if a householder planning application in London receives significant third-party objections (from neighbours, from amenity societies, or from the LPA's conservation officer in a sensitive conservation area case), the case officer may not be able to grant delegated approval and may instead refer the application to the Planning Committee for determination. A committee determination typically adds 4-8 weeks to the timeline (because the next available committee date must be awaited). Total time from application submission to planning decision: in a straightforward case (no third-party objections, no conservation area sensitivity, no pre-application concerns outstanding): 8-10 weeks from submission. In a more complex or contested case: 12-20 weeks from submission.

Once planning permission is granted, the project moves into the pre-construction phase — the detailed technical design, structural engineering, Building Regulations application, and party wall process. This phase is often underestimated by homeowners, who sometimes expect to be on site within weeks of planning approval. In practice, the pre-construction phase typically takes 10-16 weeks. Technical design development (weeks 20-26): the architect develops the planning drawings into a full technical drawing package — detailed plans, sections, and elevations with dimensions, materials specifications, M&E (mechanical and electrical) requirements, insulation specifications, window and door schedules, and any specialist details required. This is the drawing package that will be used for Building Regulations submission, contractor tendering, and on-site construction. Structural engineering design (weeks 22-28): the structural engineer designs the foundation, steel beams, structural openings, and any specialist structural elements. The structural engineer produces calculation drawings and a structural specification. Building Regulations application (weeks 24-28): the Full Plans application (recommended for extensions) is submitted to the Local Authority Building Control or a private RBCA with the technical drawings and structural calculations. The Building Control body checks the drawings for compliance with the Building Regulations and issues formal approval (or conditional approval with amendments). This typically takes 5-8 weeks from submission. Party wall process (weeks 20-32+): if the extension is within 3 metres of an adjoining owner's property (which is almost all party-wall cases in a London terrace or semi-detached), the building owner must serve a party wall notice under the Party Wall etc. Act 1996. The adjoining owner has 14 days to respond: if they consent, works can proceed. If they dissent (or do not respond), a party wall surveyor must be appointed and a Party Wall Award must be prepared. This process typically takes 6-12 weeks from notice service. The party wall process can be the critical path item in a London extension pre-construction programme — particularly where an adjoining owner is slow to respond or disputes the impact of the proposed works. Procurement (contractor selection) (weeks 26-32): the architect sends the technical drawing package to 3-4 contractors for competitive tender. Each contractor has 3-4 weeks to price the project. The architect and client review the tenders, clarify any queries, and select the contractor. Contracts are then prepared and signed.

The construction phase of a London rear extension typically starts 8-14 months after the initial design brief, depending on the complexity of the pre-construction process. Construction duration for a London rear extension: Single-storey rear extension (kitchen-diner, 20-30 sq m): 14-20 weeks on site. This includes: foundation excavation and concrete (2 weeks); ground slab (1 week); masonry walls and steels (4-6 weeks); roof structure and waterproofing (2-3 weeks); windows and doors (1-2 weeks after frame installation); first-fix M&E (electrical, plumbing, heating: 2 weeks); insulation and plasterboard (2 weeks); plastering (1-2 weeks); second-fix M&E and joinery (2-3 weeks); kitchen fit-out, floor finishes, and decoration (3-4 weeks). Double-storey extension: typically 20-28 weeks on site, depending on scope. Loft conversion (dormer): typically 8-14 weeks on site (faster than a ground-floor extension because there are fewer foundation and drainage works). Living in the property during construction: for most London extension projects, the homeowner can remain in the property during construction — the site is enclosed, dust and noise are managed, and the structural works do not typically make the property uninhabitable. However, the kitchen is typically the most disrupted room during a kitchen extension project: homeowners should plan for 4-8 weeks without a functioning kitchen (during the demolition of the existing rear wall, structural steelwork, and construction of the new extension shell). Snagging and Building Control completion: at the end of the construction phase, the Building Control Inspector carries out a final inspection and, if satisfied, issues the Completion Certificate. Snagging (the process of identifying and rectifying minor defects before handover) typically takes 1-2 weeks. Total project timeline from initial design brief to completion: for a typical London single-storey rear extension: 12-18 months. For a double-storey extension or loft conversion: 14-20 months. These timelines assume a straightforward planning process. A contentious planning application (conservation area, third-party objections) can add 3-6 months to the pre-construction timeline.