Extension timeline: what happens week by week

Last reviewed: 2026-07-01

The quick answer

A typical single-storey rear extension in the UK runs roughly 20–30 weeks on site, preceded by 2–16+ weeks of design and approvals depending on PD vs planning route. Full planning projects often need 3–5 months before the first dig.

Programmes slip when surveys are skipped, party wall notices are late, or specifications change mid-build. Map your route with the timeline planner and read the stages below week by week.

Weeks 1–4: Design, survey and route confirmation

Answer first: This phase fixes cost and calendar — rushing it causes the expensive delays later.

Activities:

• Measured survey of the house and boundaries.
• Concept layout — kitchen-diner flow, door sizes, structural opening.
• Route decision — PD, prior approval, or full planning; consider LDC.
• Structural concept — steel position, foundation type, drain searches.

Outputs: agreed floor plan, elevations, PD compliance check or planning strategy.

Homeowner actions: decide must-haves (bi-fold width, rooflights, UFH), share utility locations, confirm budget using the cost estimator.

Maven tip: one team for survey + route + structure reduces redraw loops — see build-only vs design & build.

Weeks 3–8 (PD) or 8–20+ (planning): Approvals and party wall

PD / LDC path: Submit LDC if wanted (8-week statutory period). Some homeowners build after drawings are complete without waiting — risk trade-off.

Prior approval: Notice plus 42-day process for larger rear extensions — see GOV.UK guidance.

Full planning: Application, consultation, decision (8–13 weeks minimum; longer if committee or conditions). Discharge conditions before start.

Party wall: If you share a wall, serve notices early — awards can take weeks. Read our party wall checklist.

Overlap design finalisation with approvals where possible, but do not order long-lead items until route is clear.

Weeks 1–2 on site: Enablement and strip-out

Answer first: Site setup and safe demolition of the rear wall zone — not yet the “exciting” build.

• Setting up — fencing, skip, welfare, protection to existing house.
• Soft dig — expose foundations, drains, services.
• Strip-out — remove rear windows/doors, internal finishes in the work zone.
• Temporary weathering — screen house from rain.

Building control first visit often at excavation/foundation stage.

Weeks 2–6: Foundations and structure

• Foundations — trench fill or pads per engineer; may deepen if drains found.
• Brick/block to DPC and walls — cavity, insulation, lintels.
• Steel installation — rear wall removed, temporary props, permanent frame.
• Roof structure — flat, pitched or lantern; make weather-tight ASAP.

Weather and steel delivery drive this block. A weather-tight shell unlocks internal work and reduces risk to the existing house.

Weeks 6–10: First fix

• External doors/windows/bi-folds — long-lead; order after specification lock.
• Roof finish — tiles/single-ply, fascia, gutter.
• M&E first fix — cables, pipes, heating manifolds, ventilation.
• Insulation and boarding ready for control inspection pre-plaster.

Building control inspects structure, insulation and fire/compartmentation before boarding.

Weeks 10–14: Second fix and plaster

• Plasterboard and skim — whole zone including junctions to existing house.
• Second fix electrics — sockets, switches, lighting circuits.
• Plumbing second fix — radiators/UFH commission, kitchen pipework stubs.

Moisture drying time affects decoration schedule — do not rush paint on fresh plaster.

Weeks 14–18: Kitchen, finishes and snagging

• Kitchen install — units, worktops, appliances.
• Flooring — tile, engineered wood or vinyl.
• Decoration — paint, tiling in wet areas.
• External making good — render/paint match, patio thresholds, downpipes.

This is where kitchen-diner design mistakes show up if layout was not resolved early.

Weeks 18–20: Handover and certification

• Snagging list — contractor and client walk-through; minor fixes.
• Building control completion — final certificate.
• Part P / gas safe paperwork where applicable.
• O&M info — warranties, appliance manuals, as-built drawings.

Keep LDC, party wall awards, and control certs together for resale.

What changes the timeline?

| Factor | Effect | |--------|--------| | Full planning | +2–4 months pre-build | | Party wall dispute | +1–3 months | | Custom glazing | +4–8 weeks lead time | | Ground/drain surprises | +1–2 weeks foundations | | Specification changes | Redesign + reorder delays |

Compare PD vs planning timelines on projects — e.g. Sevenoaks PD rear vs Woolwich full planning.

Next steps

• Model your programme: timeline planner.
• Sense-check route: PD Quick Check.
• Budget alignment: cost estimator and rear extension costs.
• Book a consultation for a property-specific programme after survey.