How Long Does a London Home Extension Take? A Realistic Timeline

The pre-construction phase of a London extension project is typically the longest phase — often longer than the actual build programme. The pre-construction phase covers: brief development and design; pre-application planning advice; planning application; Building Regulations application; structural engineer's design; contractor tender and appointment; and procurement of long-lead items. Brief development and concept design (weeks 1-4): the initial meetings with the architect to develop the brief (what the homeowner wants to achieve), the concept sketches, and the initial planning feasibility assessment. This phase is typically 3-6 weeks for a single-storey rear extension and 4-8 weeks for a more complex project. Pre-application planning advice (weeks 4-8): if pre-application advice is sought (recommended for any project in a conservation area, or for a two-storey, wrap-around, or unusual extension) — submitting the pre-app fee and documentation, waiting for the LPA's response, and incorporating the feedback into the design. Most London Boroughs have a pre-app response time of 4-8 weeks. Planning application (weeks 8-22): once the design is finalised, the planning application is submitted. The statutory determination period is 8 weeks (LPA target), but many London Borough householder applications take 10-13 weeks to determination. Add 2 weeks for the architect to finalise the planning drawings and submit: total planning phase approximately 12-16 weeks. In a conservation area or for a listed building application, allow 16-20 weeks for determination. Building Regulations application (weeks 12-24): the structural engineer's design and calculations are typically submitted for Building Control approval alongside the planning application (so these run in parallel). Full Plans approval from Building Control: 5-8 weeks from valid submission. Total: approximately 8-12 weeks from instruction of the structural engineer to BR approval, if running in parallel with planning. Contractor tender and appointment (weeks 20-28): the contractor tender process (inviting 3-4 contractors to quote against the specification, evaluating the tenders, negotiating with the preferred contractor, and signing the contract) takes 4-8 weeks in a well-managed process. Procurement of long-lead items (weeks 20-30): windows, bifold/sliding doors, and roof lanterns typically have 4-8 week manufacturing lead times after order. If orders are placed immediately after planning approval, the items arrive on site at approximately the right time for installation. Typical pre-construction phase total: single-storey rear extension (PD or prior approval): 16-24 weeks from architect instruction to site start. Double-storey or planning-permission-required extension: 22-36 weeks from architect instruction to site start. Listed building or conservation area extension: 28-44 weeks from architect instruction to site start.

Once the contractor starts on site, the construction programme for a London extension runs faster than the pre-construction phase. But it is not as fast as homeowners sometimes imagine — because the programme includes drying times (concrete curing, plaster drying, adhesive setting), inspection hold points (Building Control inspections at defined stages), and sequential dependencies (plastering cannot start until first-fix is complete; flooring cannot start until plastering is complete and the extension is dry). On-site programme by extension type: Single-storey rear extension (up to 20m², flat roof, standard specification — kitchen connection, bifold doors, flat roof lantern): weeks 1-2: Groundworks and foundations (excavation, steel reinforcement, concrete pour, drainage runs to the main sewer). Building Control foundation inspection required before concrete is poured. Weeks 3-5: Masonry (external brickwork walls to DPC level, DPC installation, backfill and compaction, masonry walls to roof plate level). Structural steelwork (beam installation over rear opening — 1-2 days for beam and padstones plus temporary works). Weeks 5-7: Roof structure and flat roof waterproofing (timber joists, OSB decking, VCL, PIR insulation warm roof, EPDM single-ply membrane, perimeter details, roof outlets, lantern installation). Weeks 7-9: First-fix mechanical and electrical (first-fix electrical circuits, underfloor heating mat if specified, first-fix plumbing to kitchen, gas point repositioning). Windows and doors installation (bifold doors or sliding doors — pre-ordered and arriving on schedule). Weeks 9-11: Insulation and airtightness (wall insulation batts, airtightness tape at all junctions, wall plasterboard on dabs). Weeks 11-14: Plastering and drying (skim plaster to all new surfaces — minimum 2-3 weeks drying time before decoration and flooring). Weeks 14-16: Second-fix (electrical second-fix, plumbing second-fix, joinery second-fix, floor finishes, tiling). Kitchen installation (if the kitchen is being reconfigured — allow 1-2 weeks for kitchen installation). Weeks 16-18: Decoration and completion. Snagging. Building Control final inspection and Completion Certificate. Total on-site programme for a single-storey rear extension: 16-20 weeks. Medium wrap-around extension (30-35m²): 20-26 weeks. Double-storey rear extension (35-45m²): 22-30 weeks. Loft conversion (standard): 12-18 weeks. Large kitchen extension with wrap-around (45-60m²): 26-36 weeks.

The most common causes of programme delay in a London home extension project — and how to reduce their impact: Planning determination delay (exceeds 8-week statutory target): some London Boroughs are consistently slow to determine householder applications. The most effective mitigation is submitting a high-quality application (complete drawings, well-prepared design statement, all required supporting information) to reduce the chance of the application being delayed by the LPA requesting further information. Structural engineer's design delay: if the structural engineer's calculations and drawings are not issued to the contractor before the structural phase begins, the contractor cannot order the structural steel or set out the beam pocket positions. Order the structural engineer before the planning application is even determined — structural design running in parallel with planning determination saves 4-6 weeks on the programme. Long-lead item delivery delay: if bifold doors, roof lanterns, windows, or structural steel are not ordered early enough, they arrive after the contractor needs them — causing programme stalls. Order all long-lead items immediately after planning consent is granted (even before the structural engineer's drawings are fully complete, the approximate sizes are typically known at planning stage). Rain delays during groundworks: London's rainfall is modest compared with most of the UK, but sustained wet weather in autumn or winter can delay foundation excavation and masonry work. Allow a weather contingency of 2-3 weeks in any programme scheduled through November-February. Homeowner-caused delays: the most avoidable source of delay is the homeowner's own decisions being made too late: material selection not finalised (the contractor cannot order tiles, floor finishes, or kitchen units without the homeowner's selection); additional-work decisions during construction that require redesign or re-ordering; and stage payment delays that reduce the contractor's resource to the project. Make all material selections before the contractor starts on site — keep a decision log and a procurement tracker. Ground condition surprises: in London Clay, unexpected tree roots, buried archaeological material, or uncharted drainage can cause foundation design changes that delay the groundworks phase by 2-4 weeks while the structural engineer revises the foundation design. A ground investigation (SI) before the structural design is finalised can identify these issues before they cause site delays — cost: £500-£2,000 for a desk study and limited trial pit investigation.

Combining the pre-construction and construction phases, the following are realistic total timelines from first architect appointment to construction completion for London home extensions: Single-storey rear extension (PD, no planning required): 20-30 weeks from architect instruction to construction completion. Typical range: 5-7 months. Single-storey rear extension (planning permission required): 28-40 weeks. Typical range: 7-10 months. Prior approval (Neighbour Consultation Scheme) rear extension: 22-34 weeks. Typical range: 5-8 months (shorter planning stage than full application). Double-storey rear extension (full planning required): 32-50 weeks. Typical range: 8-12 months. Wrap-around extension (full planning required): 34-52 weeks. Typical range: 8-13 months. Loft conversion (PD or planning): 18-28 weeks. Typical range: 4-7 months. Listed building or conservation area complex extension: 52-78 weeks. Typical range: 12-18 months (significantly longer due to LBC process, pre-application, and specialist construction). Key message for homeowners: if a London extension project is going to be used for a specific milestone (a new baby due in 10 months, a family event, a school term start), plan backward from that milestone with a realistic total timeline — and start the architect appointment 12+ weeks before you expect construction to begin, not 4-6 weeks. The planning process alone takes 10-16 weeks for most full planning applications. The best advice is always to start earlier than feels necessary.