How Long Does a House Extension Take? A Realistic Timeline

A house extension project has two fundamentally different phases:

**Pre-construction** (typically 3–6 months, sometimes longer): This is everything that happens before work starts on site. Design, planning, structural engineering, Building Regulations, party wall, and contractor appointment. This phase is largely outside your control in terms of speed — it depends on council processing times, neighbour responses, and design iteration.

**On-site build** (typically 8–22 weeks depending on type): Once all pre-construction approvals are in place and the contractor is mobilised, the physical build begins. This phase is more predictable, though site conditions, procurement lead times, and unforeseen discoveries can extend it.

Total project time from first meeting to completion typically ranges from **5 months** (for a straightforward single-storey rear extension using Permitted Development) to **12–18 months** (for a double-storey extension requiring full planning permission and a complex structural package).

Before any applications are submitted, you need drawings. A typical design stage for a house extension includes:

• •Initial design consultation and brief development: 1–2 weeks
• •Concept layouts and review: 1–2 weeks
• •Planning drawings or Permitted Development pack preparation: 2–4 weeks
• •Technical drawings for Building Regulations (often later, once planning is resolved): 2–3 weeks additional

• The time this takes depends heavily on:
• •How clearly defined your brief is at the start
• •How many rounds of revisions are needed
• •Whether you are using an architect, architectural technician, or design-and-build contractor

If you are using a design-and-build contractor like RCB, design and build are coordinated from day one, which typically compresses the overall programme compared with separate appointments.

Once design drawings are ready, you submit to the council. The timeline depends on the route:

• **Permitted Development + Lawful Development Certificate (LDC)**
• •Applications are submitted to the council alongside the design drawings
• •Statutory period: 8 weeks
• •In practice, most LDCs are decided within 8–10 weeks
• •No neighbour objection process, so this is the fastest route

• **Prior Approval (Neighbour Consultation Scheme)**
• •For extensions 4–6m deep on terraced/semi-detached or 4–8m on detached houses
• •Statutory period: 42 days (6 weeks)
• •In practice, if a neighbour objects, assessment can extend to 8–10 weeks

• **Full Planning Application**
• •Statutory period: 8 weeks (13 weeks for major applications)
• •In practice, many London boroughs take 10–16 weeks
• •Neighbour consultation period: 21 days built into the statutory period
• •Conservation area applications with objections can take longer
• •Some applications are delegated (decided by an officer); others go to committee — committee decisions add 4–8 weeks

If your planning application is refused and you appeal, add 6–12 months.

The Party Wall etc. Act 1996 applies to the vast majority of London extensions. Party wall notices must be served before work starts, but they can be served in parallel with the planning stage — you do not have to wait for planning permission to serve notice.

• **Timeline if neighbours consent promptly**:
• •Serve notice: 1 week
• •14-day acknowledgement period
• •Neighbours sign Party Wall Agreement: within 14 days = you can start 14 days after serving notice
• •Minimum time: 2 weeks

• **Timeline if neighbours do not respond or dispute**:
• •If neighbours do not respond or dissent within 14 days, the dispute resolution process begins
• •Each side appoints a party wall surveyor (or agrees to use one agreed surveyor)
• •The Award (formal agreement) is prepared: typically 6–10 weeks
• •Minimum time once in dispute: 2–3 months

The most common programme risk on London extension projects is a late party wall notice. Serve notice as soon as you have your design drawings — do not wait until the week before you want to start on site.

Structural engineering can be commissioned in parallel with planning, once the design is sufficiently developed:

• •Structural calculations and drawings: 3–5 weeks for a standard extension
• •Building Regulations application submission: 1 week to prepare and submit
• •Building Control initial checks: 4–5 weeks (Full Plans route)
• •Alternatively, Building Notice: submit and start — Building Control inspects on site, no formal approval before start

For a single-storey extension, structural engineering and Building Regulations are typically not on the critical path if started promptly once design is agreed. For a double-storey extension or one with complex structural elements, the structural package can take longer.

Once all pre-construction phases are complete and the contractor is mobilised, here are realistic on-site build timelines:

• **Single-storey rear extension (standard, flat or pitched roof)**
• •Up to 3m × 4m: 8–10 weeks
• •Up to 6m × 5m: 10–14 weeks

• **Side return or wrap-around extension**
• •Standard L-shape with lantern and bifolds: 10–14 weeks
• •With full kitchen relocation, structural opening, high spec: 14–18 weeks

• **Kitchen extension (rear extension + full kitchen)**
• •10–14 weeks

• **Double-storey rear extension**
• •Standard 2-bed addition: 14–18 weeks
• •With significant structural complexity: 16–22 weeks

• **What happens on site and when**:
• •Weeks 1–2: Strip-out and demolition of existing structure
• •Weeks 2–4: Excavations and foundations
• •Weeks 3–5: Drainage and slab
• •Weeks 4–6: Structural walls and steelwork
• •Weeks 5–8: Roof structure and watertight envelope
• •Weeks 7–11: First-fix M&E (electrics, plumbing, heating)
• •Weeks 9–13: Insulation, plasterboard, plastering
• •Weeks 11–14: Second-fix M&E, kitchen/bathroom fit-out
• •Weeks 13–16: Joinery, flooring, decoration, snagging

Understanding what causes extensions to take longer than expected helps you plan around the risks:

**Late party wall notices** (adds 2–3 months to programme if dispute resolution is triggered) Avoid by: serving notice as soon as design is agreed, even before planning is resolved.

**Planning applications taking longer than the statutory period** Avoid by: using PD/LDC routes where possible; submitting complete, well-prepared applications; using a planning agent for complex or Conservation Area applications.

**Design iteration extending the pre-construction phase** Avoid by: arriving at your first meeting with a clear brief, reference images, and a realistic budget range. Decisions made early cost nothing; decisions changed late add cost and time.

**Material procurement lead times** Avoid by: confirming structural steel, windows, and glazed roof specifications before work starts on site, not during the build. Lead times for bespoke bifold doors, roof lanterns, and structural sections can be 6–14 weeks.

**Unforeseen structural discoveries on site** Plan for by: building a 2-week programme contingency for standard extensions, 3–4 weeks for older properties.

**Contractor availability** Avoid by: agreeing a start date and scheduling the contractor while you are in the pre-construction phase — not after all approvals are in place.

Here is what a realistic end-to-end programme looks like for a mid-terrace in inner London:

| Stage | Duration | Notes | |-------|----------|-------| | Design and brief | 4–6 weeks | | | LDC application | 8–10 weeks | Runs in parallel with party wall | | Party wall notice | 2–3 months | Serve early — run in parallel with planning | | Structural engineering + Building Regs | 4–6 weeks | Start once design agreed | | Contractor mobilisation | 2–3 weeks | Scaffold, skip, materials order | | On-site build | 10–14 weeks | | | Snagging and sign-off | 1–2 weeks | | | **Total** | **5–8 months** | From first meeting to completion |

For an extension requiring full planning permission (e.g. a double-storey, or any works in a Conservation Area where PD doesn't apply), add 2–4 months to account for the longer planning decision period.

Starting well — with a clear brief, early party wall notices, and a contractor who coordinates pre-construction from day one — is the single most effective way to compress the overall timeline.