Home Extension Timeline: From First Idea to Completion

The project begins with a design conversation and feasibility assessment — before any drawings are produced.

**Week 1–2: Initial brief and site visit** Your architect or design-and-build contractor visits the site, takes measurements, and discusses what you want to achieve. At this stage, the questions are: What does the client want? Is it physically possible given the plot? What are the planning constraints? What is a realistic budget?

**Week 2–4: Concept design** The architect or design team produces initial concept layouts — floor plans and perhaps an initial elevation — to test whether the brief and site work together. This is where the footprint, the height, the internal layout, and the approach to light are determined. Good design decisions at this stage prevent expensive changes later.

**Week 4–6: Feasibility confirmed** By the end of Stage 1, you should know: the approximate footprint and height of the extension; whether it falls within Permitted Development or requires planning consent; a realistic budget range; and whether a structural engineer needs to be brought in early (which for most extensions, they should be).

**What controls timing at Stage 1**: Client decision speed. If the brief is clear and decisions are made promptly, Stage 1 can be completed in 3–4 weeks. If the brief changes repeatedly or sign-off is delayed, this stage can stretch to 12+ weeks.

This is the longest and most variable stage — and the one over which you have the least control once planning is submitted.

**Weeks 6–10: Full planning drawings and application** If planning consent is required, the architect produces a full planning drawing set (existing and proposed floor plans, elevations, and site plan) and submits a Householder Planning Application to the local authority. The application fee is currently £258 for most householder extensions in England.

**Weeks 10–18: Planning determination period** London authorities have 8 weeks to determine a Householder Application (or 13 weeks with extensions). In practice, most straightforward householder extensions are determined in 8–12 weeks. Complex or contested applications may take 13–16 weeks. If refused, an appeal adds a further 6–12 months.

**Weeks 18–22: Technical design** Once planning is approved, the technical design stage produces: structural engineer drawings and calculations (beams, foundations, padstones), drainage design, insulation and U-value calculations for Part L compliance, electrical and plumbing layout, and full specification of finishes and materials.

**Weeks 20–24: Building Regulations application** A Full Plans application (or Building Notice) is submitted to Building Control (local authority or an Approved Inspector). Full Plans approval typically takes 5–8 weeks from submission. Building works cannot start until Building Control has reviewed the application (or a Building Notice has been submitted).

**Permitted Development note**: If the extension falls within Permitted Development, Stage 2 is compressed significantly — technical design and BC application can proceed in parallel, and the planning element is replaced by a straightforward Lawful Development Certificate application (typically 6–8 weeks, compared to 8–12 for planning).

Once planning and Building Regs are in place, the project moves to contractor procurement and site preparation.

• **Weeks 22–26: Contractor tendering and appointment**
• If you are using a separate architect and contractor (rather than a design-and-build contractor), this is the stage at which you invite contractors to tender against the completed drawings and specification. A proper tender process involves:
• •Invitation to 3–5 contractors
• •Site visit with each tenderer
• •2–3 weeks for tender preparation
• •Review, comparison, and commercial negotiation
• •Contractor appointment and contract execution

With a design-and-build contractor, this stage is often condensed — the contractor who designed the project also builds it, so there is no separate tender process.

• **Weeks 26–28: Pre-start programme and procurement**
• Once the contractor is appointed, the following should be in place before work starts:
• •Programme — detailed week-by-week schedule with milestone dates
• •Subcontractor appointments — groundworker, structural steel (if required), plumber, electrician, plasterer
• •Material procurement — structural components (steel beams, structural lintels), windows and doors ordered (8–12 week lead time for aluminium windows is common)
• •Party wall matters — if required, party wall award must be in place before excavation
• •Site welfare — toilets, hoarding, site security arrangements

The construction stage is what most clients think of as 'the project' — but as the timeline above shows, it is only part of the journey. For a standard single-storey rear extension, the construction programme typically runs 10–14 weeks.

• **Weeks 1–3 on site: Enabling, demolition, and groundworks**
• •Demolition of existing structure (rear wall, conservatory, outbuilding) where required
• •Excavation of foundations to structural engineer's specification (depth depends on soil conditions and proximity to trees)
• •Pouring strip or pad foundations
• •Laying insulation below slab (PIR boards)
• •Pouring ground floor slab
• •Building Control stage inspection: foundation inspection (before backfilling) and oversite inspection (before slab pour)

• **Weeks 3–6 on site: Superstructure**
• •Masonry or timber frame walls
• •Structural steel installation where required (padstones, beams, temporary props)
• •Flat or pitched roof structure
• •Weathering — felt and battens on pitched roof, or flat roof waterproofing membrane
• •Window and door openings formed; lintels installed

• **Weeks 6–10 on site: First fix**
• •First-fix plumbing (soil and waste pipes, UFH manifold and pipe loops if specified, hot and cold feeds)
• •First-fix electrics (cable runs, back-boxes, socket and switch positions)
• •Insulation — between rafters, under flat roof, cavity walls
• •Building Control inspections: structural frame, insulation before plasterboard

• **Weeks 10–14 on site: Second fix and finishes**
• •Plastering (wet plaster or plasterboard-and-skim)
• •Second-fix plumbing (sanitary ware, radiators or UFH commissioning)
• •Second-fix electrics (sockets, switches, consumer unit upgrade where required)
• •Windows and external doors installed (if not already)
• •Floor screed or floor finish (tiles, engineered wood)
• •Painting and decorating
• •Final snagging and handover
• •Building Control final inspection and completion certificate issued

The total project timeline is the sum of each stage — and delays at any stage push the start date for every subsequent stage.

**Planning delay**: The most common cause of projects taking longer than clients expect. Local authority decisions are outside your control once the application is submitted. Incorrect drawings, missing information, or neighbour objections can add 4–12 weeks to the planning stage.

**Design change after planning**: Any material change to the design after planning consent is granted requires an amendment application — adding 4–8 weeks. Avoid this by finalising the design thoroughly before submission.

**Windows and doors lead time**: Aluminium windows and bifold/sliding doors often have 8–12 week lead times. If ordered too late in the programme, the structure can be built but watertightening is delayed.

**Structural engineer delays**: Structural drawings and calculations are required before Building Regs can be submitted and before steelwork can be fabricated. Delays in appointing the engineer or in providing survey data compress subsequent stages.

**Party wall**: If you share a wall with a neighbour, party wall procedures must be served and an Award agreed before excavation. If neighbours dissent, the process adds 4–8 weeks. A surveyor-managed party wall award takes typically 6–10 weeks from first service of notice.

• **Realistic total timeline summary**:
• •Single-storey rear extension (planning required): 9–14 months
• •Single-storey rear extension (Permitted Development): 7–10 months
• •Double-storey extension: 12–18 months
• •Loft conversion + rear extension combination: 14–20 months