London Home Extension Timeline: How Long Does Each Stage Take?

From the decision to proceed to receiving the initial design drawings from your architect or design-and-build contractor: Initial meeting and brief: 1-2 weeks from contact to first meeting. Scheme design (concept layout, initial elevations, outline specification): 2-4 weeks from initial meeting. Design review and client feedback: 1-2 weeks. Scheme design approval: total 4-8 weeks from first appointment to agreed scheme design. What affects this stage: the clarity of the brief — a clear, well-defined brief produces design proposals faster than a brief that evolves significantly through multiple rounds of revision. Time to review: if the homeowner is unavailable to review designs promptly, turnaround time extends. Complexity: a straightforward single-storey rear extension with no conservation area complications moves faster than a complex conservation area project where the design must carefully address heritage concerns.

From the agreed scheme design to planning decision or PD confirmation: Permitted development — no application required: if the extension is clearly within PD limits and no LDC is sought, no planning stage is needed — zero weeks. Lawful Development Certificate (LDC) application: 8-10 weeks for determination by the Local Planning Authority. Prior approval (Larger Home Extension scheme): 8-10 weeks from submission (42-day neighbour consultation period, then decision). Full planning permission application: 8-12 weeks statutory determination period (10-14 weeks in practice for most London Boroughs, due to workload). Conservation area full planning application: 10-16 weeks in practice — conservation officer consultations and the risk of committee referral add time. Planning refusal and appeal: 28-52 weeks from refusal to appeal decision — a major programme risk. What causes planning delays in London: incomplete application (missing plans, incorrect fee): the LPA will not validate an incomplete application — validation takes 1-2 weeks, and an incompletely submitted application restarts the clock. Committee referral: if the application is contentious (many objections, heritage significance), it may be referred to the planning committee rather than decided under delegated authority — adds 4-8 weeks. Officer caseload: some London Boroughs have significant planning officer caseloads — applications may not be allocated to an officer until 3-4 weeks after validation.

Running in parallel with planning (ideally): Party wall notice service: the notice can be served immediately the scheme design is agreed, without waiting for planning permission. 2-month notice period for most party wall matters (1 month for some). Party wall award: if the neighbour consents, no award is needed. If the neighbour does not respond or dissents: a party wall award takes 4-8 weeks to prepare and agree (if an agreed single surveyor) or 8-16 weeks (if each party appoints their own surveyor and an umpire is required). Structural engineer appointment and calculations: appointment: 1-2 weeks. Foundation design: 2-3 weeks from site investigation data. Steel beam design and calculations: 2-4 weeks. Building Regulations Full Plans application: submission and validation: 1 week. Assessment and approval (for a standard residential extension): 4-6 weeks. Note: if the Building Regulations application is submitted during the planning process (rather than after planning approval), the Building Regulations can be approved before planning — allowing construction to start sooner after planning approval. Running these parallel stages simultaneously saves 4-8 weeks on the overall programme.

From Building Regulations approval (or parallel with it) to contractor appointment: Tender document preparation (drawings, specification, schedule of works): 1-2 weeks. Tender invitation and tender period: 2-4 weeks for contractors to price the project. Tender return and analysis: 1-2 weeks. Contractor interviews and clarifications: 1-2 weeks. Contract negotiation and execution: 1-2 weeks. Total tender and appointment stage: 4-8 weeks. What affects this stage: number of tenderers — inviting 3 rather than 5 contractors simplifies the process; responding to tender queries promptly keeps the programme moving. Contractor availability — a quality contractor may have a 4-8 week lead time between appointment and start on site. Programme pressure — avoid setting an unrealistic start date that leads to appointing the wrong contractor; the tender process should identify the best contractor for the project, not merely the fastest to start.

Construction duration for common London extension types: Single-storey rear extension (15-20m²): 10-14 weeks. Single-storey rear extension with full ground floor refurbishment: 14-22 weeks. Double-storey rear extension: 16-24 weeks. Loft conversion (rear dormer): 12-18 weeks. Hip-to-gable plus rear dormer loft: 14-20 weeks. What causes construction programme overruns in London: Late material deliveries: specialist items (bifold doors, rooflights, bespoke joinery) with long lead times that were not ordered early enough. This is the most common cause of London extension construction delays — order all specialist items at the start of the construction programme. Unforeseen conditions: unexpected ground conditions (buried structures, old drainage, difficult soil), unforeseen structural conditions (inadequate existing structure, asbestos discovery, unexpectedly shallow foundations). Weather: significant delays are uncommon in London for structural works (which can proceed in most weather), but extreme weather (prolonged frost, high winds during roof works) can cause delays. Subcontractor availability: if the main contractor is relying on specialist subcontractors (plumber, electrician, roofer) who are unavailable at the required time, the programme slips. Subcontractor management is a key principal contractor responsibility. Total programme summary (realistic): simple PD single-storey rear extension: 6-12 months from first appointment to practical completion. Conservation area single-storey rear extension (with planning application): 10-16 months. Double-storey extension with full planning permission: 14-22 months. Loft conversion: 8-14 months. Add 12-18 months for a planning appeal scenario.