London Home Extension Timeline: From Planning Application to Moving In

Before any formal work begins, most homeowners spend time defining what they want, researching architects, and getting initial indicative costs.

**Week 1-2: Research and definition** - Define your goals and rough budget - Research what similar projects in your area look like - Gather inspiration images and an initial wish list - Establish basic feasibility (size constraints, planning restrictions for your area, conservation area status)

**Week 2-4: Architect selection** - Meet 2-4 architects (most offer free initial consultations) - Discuss feasibility, their approach, and fee structure - Typical architect fees for a residential extension: 8-12% of build cost, or a fixed fee structure for defined stages - Appoint preferred architect and agree scope of services

**Typical duration for Stage 1: 2-6 weeks** — longer if you are still deciding on the scope or checking mortgage implications.

Once you have appointed an architect, design development covers several sub-stages:

**Concept design (2-4 weeks):** - Initial layout options explored - Size, form, and relationship to existing house defined - Massing and basic appearance decisions made - Client review and selection of preferred option

**Developed design (3-5 weeks):** - Detailed floor plans, elevations, and sections produced - Internal layouts refined - Material palette and key specifications discussed - Structural engineer may be brought in at this stage to advise on feasibility of structural elements

**Planning drawings produced (2-4 weeks):** - Final planning drawings prepared (floor plans, elevations, site plan, existing plans for comparison) - Design and Access Statement prepared where required (usually for planning applications in conservation areas or listed buildings) - Pre-application consultation with the LPA (recommended for more complex schemes) may add 4-8 weeks but significantly reduces planning risk

**Typical duration for Stage 2: 6-14 weeks** — shorter for simpler projects within Permitted Development, longer for bespoke designs or schemes requiring pre-application consultation.

Planning applications in London take between 8-16 weeks from submission to decision, with the statutory determination period being 8 weeks for most householder applications.

**Submission and validation (1-4 weeks):** - Architect submits the planning application to the London Borough - The LPA validates the application (checks it is complete) — this takes 1-3 weeks - The 8-week determination clock starts from the validation date (not the submission date)

**Consultation and assessment period (8 weeks statutory):** - Neighbours and statutory consultees are notified - A planning officer is assigned and assesses the application - If the application is contentious, the officer may request amendments to overcome objections — this can add 4-8 weeks to the process

**Determination:** - Most straightforward householder applications in London are determined in 8-12 weeks - Contentious applications, committee cases, or those with pre-commencement conditions can take 12-20 weeks - Pre-commencement conditions (requiring the discharge of conditions before work starts) can add another 4-8 weeks after approval before work can begin

**Permitted Development alternative:** - If your project qualifies as Permitted Development (many rear extensions and loft conversions do), no planning application is needed - However, a Certificate of Lawful Development (CLD or CLOPUD) is recommended before starting — this takes 8 weeks and confirms the works are lawful - Total time for PD route including CLD: approximately 8-10 weeks

**Typical duration for Stage 3: 8-16 weeks for planning; 8-10 weeks for PD + CLD route**

Even after planning approval, there is significant work to be done before building can start.

**Building Regulations drawings (4-8 weeks):** - Detailed technical drawings produced for Building Control submission - Structural engineer produces structural design, calculations, and specifications - If a Full Plans application is made (preferred), Building Control must approve the drawings before work starts — this takes 4-8 weeks - If a Building Notice is used instead (for simpler projects), no pre-approval is needed but the drawings must still be prepared

**Structural engineer design (concurrent with above): 2-4 weeks** - Structural calculations for foundations, steelwork, and new floor/roof structures - Engineer attends site for a site visit before producing final calculations

**Party wall process (where applicable): 2-4 months** - Party Wall Notices must be served 2 months before work starts (if the neighbour consents) - If the neighbour does not respond within 14 days or dissents, a Party Wall Award must be agreed — this process can take 2-4 months and must be complete before structural work to the party wall begins - Always serve Party Wall Notices as early as possible — ideally at the same time as submitting the planning application

**Contractor selection and tender (4-8 weeks):** - Detailed specification and bill of quantities prepared by the architect or QS - Tender package sent to 3-5 contractors - Tenders received and compared - Preferred contractor appointed and contract agreed - Programme and start date agreed with contractor

**Typical duration for Stage 4: 10-18 weeks** — the main variables are party wall complexity and how quickly a suitable contractor can be appointed and a start date agreed.

The build phase is typically the shortest part of the overall project timeline — the misconception that building work takes the longest is usually wrong. Most of the time is spent in the earlier stages.

**Rear single-storey extension build timeline:** - Groundworks and foundations: 2-4 weeks - Masonry and structural frame: 3-5 weeks - Roof structure and weathertight: 2-4 weeks - First fix M&E and internal frame: 2-4 weeks - Plastering: 1-3 weeks - Second fix, kitchen/bathroom, finishes: 3-6 weeks - Snagging and completion: 1-2 weeks - **Total: 14-28 weeks depending on size and complexity**

**Two-storey extension or L-shaped extension:** - Additional 4-8 weeks for the additional floor and associated works - **Total: 18-34 weeks**

**Factors that cause build programme delays:** - Bad weather (external groundworks and masonry cannot proceed in severe frost) - Unforeseen site conditions (unexpected drainage runs, unknown foundations, asbestos) - Material lead times (bespoke windows, steelwork) - Building Control hold points requiring inspection before work proceeds - Client change requests mid-build - Subcontractor availability gaps in the programme

Adding all stages together for a typical London rear extension requiring planning permission:

- Stage 1 (concept and appointment): 2-6 weeks - Stage 2 (design development): 6-14 weeks - Stage 3 (planning application): 8-16 weeks - Stage 4 (technical design and pre-construction): 10-18 weeks - Stage 5 (build): 14-28 weeks

**Total: 40-82 weeks (approximately 10-20 months)**

For a Permitted Development project (where planning permission is not needed), the total shortens to approximately 8-16 months.

**Common reasons the timeline extends beyond these estimates:** - Architect design changes after client review - Planning amendments requested by the LPA - Pre-commencement planning conditions taking time to discharge - Party wall delays - Contractor programme gaps (the best contractors in London are often booked 3-6 months ahead) - Material lead times for bespoke items - Client decision delays on finishes and specifications

**Planning realistic milestones:** - From first architect meeting to build start: 12-18 months is typical for a planning-required extension - From build start to moving back in (for occupied houses undergoing refurbishment and extension simultaneously): allow an extra 2-4 months beyond the bare build programme for the chaos of live occupation during works

The homeowners who manage best are those who start the pre-construction process 18-24 months before they want to be living in the finished space.