Home Extension Timeline: From Brief to Handover

Introduction

One of the most common questions homeowners ask at the start of a home extension project is: how long will this take? The honest answer depends on the complexity of the project, the planning context, the contractor programme, and many factors outside anyone's control. But there is a typical sequence of stages and duration ranges that allow a realistic programme to be understood from the outset. This guide explains the typical timeline for a residential extension project in north London — from the initial brief through to practical completion and handover — and identifies the key factors that affect duration at each stage.

Stage 1: Brief and Feasibility (1–2 Months)

The first stage involves defining the project brief with the architect and carrying out a feasibility assessment. This includes:

• Site visits and initial discussions to understand the homeowner's requirements and aspirations
• Review of planning constraints — conservation area status, Article 4 Directions, permitted development rights, neighbouring consents
• Title and covenant review (typically conducted by the homeowner's solicitor in parallel)
• Early cost discussion — ballpark cost ranges for the project type to ensure the brief is viable within the budget
• Appointment of the architect and agreement of the scope of services

For a standard rear extension in a non-conservation area, feasibility can be straightforward. For a complex project (basement, conservation area, listed building) this stage may require preliminary discussions with the planning authority (pre-application advice service) which extends the duration.

Stage 2: Concept and Developed Design (2–3 Months)

The architect develops the design through sketch and concept design (RIBA Stage 2) and then developed design (RIBA Stage 3), incorporating specialist inputs from the structural engineer, M&E consultants, and any other advisors:

• Developed floor plans and elevations defining the spatial arrangement and external form
• Structural concept developed with the engineer — key structural elements identified and positioned
• M&E system design — heating, ventilation, electrical and plumbing strategy agreed
• Planning drawings prepared (if planning permission is required)
• Pre-application advice meeting (if needed) — typically 6–10 weeks to obtain a written response from the LPA

Stage 3: Planning Application (8–16 Weeks)

For projects requiring planning permission, the planning application is submitted at the end of the design development stage and the decision period is determined by the Local Planning Authority:

• Householder planning applications: 8-week statutory determination period. In practice, Camden, Islington and Barnet typically determine within 8–10 weeks for straightforward applications; contentious applications may take longer, go to appeal, or be referred to committee.
• Listed building consent and conservation area consent: 8-week determination period; may be referred to Historic England for Grade I and II* listed buildings
• Pre-application advice: adds 6–12 weeks before the formal application stage but reduces the risk of refusal and associated delays

For projects within permitted development rights, no formal planning application is required and this stage is replaced by the shorter lawful development certificate (LDC) process if confirmation is needed (4–8 weeks).

Stage 4: Technical Design and Building Regulations (2–3 Months)

Following planning approval, the design is developed to technical design stage (RIBA Stage 4) — the detailed working drawings and specification from which the building can be constructed and from which contractors can tender:

• Full structural drawings and calculations produced by the structural engineer
• Detailed architect's drawings — sections, details, finishes schedules
• MEP (mechanical, electrical, plumbing) design and specification
• Building Regulations submission — Full Plans application to the local authority or an approved inspector. Full Plans approval typically takes 5–8 weeks.
• Party wall notices served on adjoining owners (if applicable) — a minimum 1-month notice period, with Award process if consent is not forthcoming adding a further 1–2 months

Stage 5: Tender and Contractor Selection (1–2 Months)

The tender process — inviting priced bids from contractors against the technical design package:

• Preparation of tender package: drawings, specification, form of contract, preliminaries
• Invitation of tenders to 3–5 pre-qualified contractors: typically 3–4 weeks tendering period
• Tender analysis, clarifications and negotiation: 2–3 weeks
• Contractor appointment and contract execution: 1–2 weeks

The contractor selection process should not be rushed — selecting the right contractor on the basis of price, quality and programme is a key decision that affects the entire construction stage.

Stage 6: Construction (3–18 Months)

Construction duration depends primarily on project scale and complexity:

• Single-storey rear extension (30–50 sqm): typically 12–20 weeks on site
• Rear extension with loft conversion: typically 20–30 weeks on site
• Rear extension with basement: typically 9–18 months on site
• Whole-house renovation with extension: typically 12–24 months on site

The key construction stages are:

1. Site establishment, demolition and excavation
2. Foundations and substructure
3. Superstructure (walls, roof, structure)
4. Weathertight — windows, doors, roofing complete
5. First fix (structural, M&E, insulation)
6. Plasterboard, plastering, screed
7. Second fix (joinery, M&E fitout)
8. Finishes (tiling, decorating, flooring)
9. Snagging and completion

Stage 7: Practical Completion and Defects Liability Period

The Practical Completion Certificate is issued when the works are complete and the property is ready for occupation. Following practical completion, the Defects Liability Period (typically 12 months) begins, during which the contractor returns to rectify any defects arising from defective workmanship or materials. The Making Good Certificate is issued at the end of the DLP.

Total Project Duration: Summary

Conclusion

A home extension project in north London typically takes between one and two and a half years from initial brief to practical completion, depending on the project type, planning context and construction complexity. Understanding this timeline from the outset allows homeowners to plan their own schedule — whether they need to complete before the arrival of a new family member, the end of a tenancy, or some other deadline. An architect experienced in north London residential projects will be realistic about programme from the very beginning, identifying the likely planning risks and construction complexities that could extend the timeline, and managing the project sequence to minimise unnecessary delay at each stage.