The Complete Stages of a London House Extension: From Idea to Completion in the Right Order

**Stage 1: Design concept and early feasibility (Weeks 1–4)**:

• *What happens*: the first stage is establishing what is feasible on your specific property — what type of extension, what size, and whether planning permission will be needed. This typically involves:
• •Appointing an architect or design-and-build contractor for early feasibility advice
• •A site visit to assess the property's existing layout; the rear and side garden space; the relationship with the party walls; drainage; and the relationship with the street
• •A measured survey of the existing property (either by the architect or by a specialist measured survey company) — essential for producing accurate existing drawings
• •Concept layout options — typically 2–3 options are explored at this stage, reviewing different extension footprints and configurations
• •Early feasibility check on planning status: is the proposal permitted development, or will planning permission be required?
• •Early cost guidance: a ballpark figure can be provided at this stage based on the likely scope

*Key decision at Stage 1*: which design direction to proceed with; the target extension type (single-storey rear; side return; wrap-around; double-storey); and whether to proceed on a design-and-build basis (one contract with a principal contractor who manages design and construction) or separate architect and contractor appointments

*Timeline*: 2–6 weeks, depending on how quickly the measured survey is completed and design options are reviewed

**Stage 2: Planning application (or PD confirmation) (Weeks 4–16)**:

• *What happens*:
• •If the works are **permitted development (PD)**: the architect confirms PD compliance and may prepare a Lawful Development Certificate (LDC) application for formal confirmation from the LPA. LDC application timeline: typically 8 weeks. Cost: £129 application fee + architect fee (£800–£1,500)
• •If planning permission **is required**: the architect prepares full planning drawings (existing and proposed plans, sections, elevations; site location plan; block plan; design and access statement if needed); any required supporting surveys are commissioned (arboricultural; bat; heritage; flood risk — see `planning-permission-costs-guide`); pre-application advice from the LPA may be sought first (£200–£600 + 4–6 weeks)
• •Planning application is submitted to the LPA
• •LPA validates the application (typically 1–2 weeks after submission — may be longer if documents are missing or incorrect)
• •Application is publicly advertised by the LPA (14 days for neighbour notification; 21 days for site notice)
• •LPA officer assesses the application; may request additional information from the architect during the assessment period
• •LPA issues decision (Grant or Refuse)

*Planning application timeline*: 8 weeks target from validation for a householder application. Allow 10–14 weeks in total from submission for most London Borough applications (some Boroughs are consistently slower). Prior Approval (Larger Home Extension): 42 days from notification

*Key decision at Stage 2*: whether to appeal if refused (or consider amending the design and re-applying); whether to appoint a planning consultant for contentious applications; which surveys are required

**Stage 3: Detailed technical design (Weeks 8–18 — typically running concurrently with planning from Weeks 8 onwards)**:

While the planning application is being assessed, the detailed technical design work can begin — but should not be finalised until planning permission is confirmed (as planning conditions or officer comments may require changes to the design):

• *Technical design scope*:
• •**Architectural technical drawings** (production drawings; specification; door and window schedules; finishing schedules; materials specification for Building Regulations submission)
• •**Structural engineering design** (by a Structural Engineer): foundation design; RSJ beam sizing and padstone details; temporary propping design; specification of structural elements. Fee: typically £1,200–£3,500 for a standard London extension structural design package
• •**M&E design** (mechanical and electrical): if the extension includes significant M&E changes (new boiler; underfloor heating; rewire; new distribution board) — M&E engineers may be appointed at this stage or the information may be developed by the contractor during construction. For a standard kitchen-diner extension, M&E design is typically contractor-led with architect coordination

*Timeline*: allow 4–8 weeks for structural engineering design; 4–8 weeks for full architectural production drawings. Can run concurrently with the planning determination period

**Stage 4: Building Regulations application (Weeks 10–20)**:

Building Regulations approval is mandatory for house extensions — it confirms compliance with the technical standards set out in Parts A–R of Schedule 1 to the Building Regulations 2010. Two application routes:

• *Route 1 — Full Plans application*:
• •Detailed architectural and structural drawings submitted to the LPA's Building Control department (or to an approved inspector — Approved Inspectors provide an alternative to LPA Building Control)
• •Building Control checks the full drawings for compliance with all Parts (A — Structure; C — Site preparation; E — Sound; F — Ventilation; L — Energy efficiency; P — Electrical; etc.)
• •Building Control issues a **full plans approval** (conditional or unconditional) — typically 5 weeks from submission
• •Advantage: full technical approval before construction starts; contractor can tender with confidence; resolves compliance issues before rather than during construction
• •Cost: typically £400–£900 LPA Building Control fee for a standard extension (fees vary by LPA)

• *Route 2 — Building Notice*:
• •A simple notification to Building Control that work is commencing — no detailed drawings submitted upfront
• •Building Control inspects at defined stages during construction
• •Advantage: faster to start on site; less paperwork
• •Disadvantage: compliance issues discovered during construction (requiring rectification at cost and delay); no upfront certainty; unsuitable for complex projects

*Recommendation for London house extensions*: Full Plans application is strongly recommended for all but the simplest of works — it provides certainty, reduces construction-stage risk, and is required by most mortgage lenders for any extension

**Stage 5: Party Wall Notices and Awards (Weeks 1–16 — must start early)**:

The Party Wall etc. Act 1996 is a separate process that runs in parallel with planning and Building Regulations — but it has its own minimum notice periods that mean it must be started EARLY:

• *Party Wall Notice periods*:
• •**Section 3 Notice** (work to an existing party wall): minimum **2 months notice** before works commence
• •**Section 6 Notice** (foundation within 3m of neighbour's foundation): minimum **1 month notice** before works commence
• •**Section 1 Notice** (new wall on line of junction): minimum **1 month notice** before works commence

*Consequence of starting late*: if Party Wall Notices are served only when planning is confirmed (Weeks 10–14), and the neighbour consents immediately, work can start at Week 18–20 (after the 1–2 month notice period). But if the neighbour dissents (even just to protect their interests — a very common response in London), the Party Wall Award process takes 4–10 additional weeks to complete. This can push the construction start to Week 26–30 — a significant programme impact.

*Best practice*: serve Party Wall Notices at the earliest opportunity — as soon as the extension footprint and position is confirmed (Stage 1–2), regardless of whether planning permission has been obtained. Planning permission and Party Wall Award are independent processes — you do not need planning permission before serving Party Wall Notices

*Party Wall Award timeline*: consent from all neighbours within 14 days = proceed immediately; one or more dissents = appoint party wall surveyors (2–4 weeks) + draft and agree Party Wall Award (4–8 weeks) = 6–12 weeks from notice service to award

**Stage 6: Procurement — appointing the extension contractor (Weeks 14–22)**:

*What happens*: once the design is sufficiently developed (planning confirmed; Building Regulations drawings at draft stage; structural design in progress), the extension can be tendered to contractors for pricing:

*Procurement routes for a London house extension*:

1. **Design-and-build with a principal contractor** (the RCB approach): one appointment covers design coordination, procurement, management, and delivery. The principal contractor takes responsibility for coordinating the architect, structural engineer, and specialist subcontractors. Fee structure: either a fixed price (preferred; locked before construction starts) or a cost-plus management contract. Advantages: single point of accountability; coordinated programme; reduces the homeowner's management burden

2. **Traditional procurement** (homeowner appoints architect + contractor separately): the architect produces the full drawings and specification; 3–5 contractors are invited to tender against the drawings; the homeowner appoints the lowest or best-value tender; the architect administers the contract. Advantages: competitive tendering can deliver good pricing; architect acts as the homeowner's representative during construction. Disadvantages: requires a complete drawing package before tendering (adds time); contractor selection and management is more involved for the homeowner

• *Key documents for tendering*:
• •Full architectural production drawings (or near-final)
• •Structural engineer's drawings (or outline structural specification)
• •Specification (materials; products; standards)
• •Form of contract (standard RIBA or JCT Minor Works contract recommended — not just a letter or verbal agreement)

*Getting quotes*: for a £50,000–£100,000 London extension, obtain a minimum of 3 competitive tender prices. Allow contractors 2–4 weeks to price properly. A tender period shorter than 2 weeks typically produces poorer quality prices with larger provisional sums and risk allowances

*Key decision at Stage 6*: choose the contractor; agree the contract sum; sign the contract; agree the programme and start date

**Stage 7: Pre-construction surveys, utilities, and CDM preparation (Weeks 18–24)**:

Before groundwork starts, a number of pre-construction checks and preparations are needed:

• *Utility surveys*:
• •**Drainage / sewer check**: confirm the location of all underground drains on site (foul and surface water). The extension foundation must not undercut existing drains without a Build Over Agreement (BOA) from Thames Water. Commission a CCTV drain survey (£350–£800) if drain locations are uncertain. If the extension is within 3m of a public sewer, a BOA application must be submitted to Thames Water before groundwork starts (see `build-over-agreement-guide`). BOA processing time: 2–6 weeks
• •**Gas, water, electric**: confirm utility service entry points and depths. Gas and electric services entering the building from the rear may need to be diverted if they cross the extension footprint. Utility diversions: 4–12 weeks notification and implementation, depending on the utility provider
• •**Ground investigation**: for extensions on London clay near trees, a ground investigation report (trial pits or boreholes) confirms the foundation depth and ground conditions. Typically arranged by the structural engineer. Cost: £600–£2,500 for a standard London residential site investigation

• *CDM (Construction Design and Management) 2015 preparation*:
• For a house extension where more than 1 contractor is involved (most London extensions), the CDM Regulations 2015 apply. The client (homeowner) has specific duties under CDM 2015:
• •Appoint a Principal Contractor (the main contractor building the extension)
• •Appoint a Principal Designer (the architect or design coordinator who coordinates health and safety in the design phase)
• •Ensure a Construction Phase Plan is prepared by the Principal Contractor before work starts
• •If the project lasts more than 30 working days with more than 20 workers simultaneously, or exceeds 500 person-days: notify the HSE via the F10 notification

For a standard single-storey London house extension (duration 8–14 weeks; typically 2–6 workers at peak), the F10 notification threshold is unlikely to be reached — but the CDM duties for a domestic client still apply (Construction Phase Plan; Principal Designer appointment).

• *Materials procurement*:
• For a standard London extension, the contractor procures most materials during the works. However, long-lead items should be ordered before work starts:
• •Structural steel RSJ (if required): typically 2–4 weeks from order to delivery; some specialist beams 6–8 weeks
• •Roof lanterns or large-format rooflights: typically 3–6 weeks from order to delivery for standard sizes; 8–12 weeks for bespoke
• •Bifold or sliding door systems: 4–8 weeks for standard sizes; 10–14 weeks for bespoke or powder-coated finishes
• •Roof windows (Velux; Fakro — standard sizes): typically 1–2 weeks

Order long-lead items as soon as the specification is confirmed — delays in delivery are one of the most common causes of programme overrun on London extension projects

**Stage 8: Groundwork and foundation (Construction Weeks 1–3)**:

• *What happens*:
• •Site set-up: hoarding or boarding; skip placement; scaffold design and erection (see `scaffolding-hire-london-guide`)
• •Demolition of existing structures in the extension footprint (back addition wall; outbuilding; rear garage; existing patio/decking and surface)
• •Trial pit excavation to confirm ground conditions (if not done pre-construction)
• •Foundation excavation to design depth (900–1,500mm on London clay; deeper near trees)
• •Temporary supports and propping if existing structure is being supported over the excavation
• •Laying of foundation concrete (typically C30/37 or C35/45 depending on structural design; reinforced if specified)
• •Drainage work: installation of new drainage runs; connection to existing drain; inspection chambers
• •Thames Water CCTV survey if BOA is in place

*Building Control stage inspection*: **Commencement inspection** (before foundation is poured) and **Foundation/DPC inspection** (after foundation concrete poured but before backfilling). Notify Building Control at least 24 hours before each inspection

*Timeline*: 2–4 weeks (depending on depth; drainage complexity; ground conditions; any unexpected obstructions)

**Stage 9: Superstructure — walls, structure, and roof (Construction Weeks 3–8)**:

• *What happens*:
• •Masonry walls built off the foundation (external cavity wall with insulation; party wall infill — all to the structural and architectural specification)
• •Damp-proof courses (DPCs) and cavity trays at correct positions
• •Structural opening formed through existing rear wall: temporary propping erected; masonry cut; padstones placed; RSJ beam installed and packed tight; propping removed
• •Roof structure: flat roof timber joists at engineered centres; or pitched roof cut rafters and ridge; structural beams at critical positions (as per SE design)
• •Roof covering: flat roof (EPDM rubber membrane; or GRP fibreglass; or hot melt); or pitched roof (tiles; slates)
• •Rooflights or roof lanterns installed into the roof structure
• •External windows and doors (bifold; sliding; casement) installed at this stage

*Building Control stage inspections*: **DPC/oversite** (ground slab preparation); **Frame/wall** (mid-construction); **Structural openings** (RSJ in place)

*Timeline*: 3–6 weeks

**Stage 10: First-fix services (Construction Weeks 7–10)**:

• *What happens* (while walls and roof are being completed or immediately after):
• •**First-fix electrics**: chasing; running new circuits (lighting; power; EV charger conduit if required); distribution board updates; underfloor heating wiring if UFH electric
• •**First-fix plumbing**: pipework for radiators; underfloor heating pipework laid in slab screed (if wet UFH); new boiler pipework; soil and waste pipework for new bathroom or WC
• •**First-fix ventilation**: ducting for MVHR (if included); kitchen extract ducting; bathroom extract ducting
• •Insulation installed (cavity wall partial-fill or full-fill; roof insulation above or between rafters per Part L 2021 compliance)
• •Airtightness membrane installed (increasingly required for Part L 2021 compliance — a continuous membrane across walls, ceiling, and floor at the junction between new and old)

*Building Control inspection*: **First-fix inspection** (before walls are drylinded or plastered)

*Timeline*: 2–4 weeks

**Stage 11: Second-fix and finishes (Construction Weeks 10–16)**:

• *What happens*:
• •Drylining or plastering of walls and ceilings (depending on specification — plasterboard dot-and-dab; or wet plaster; or skim over board)
• •Floor screed (if wet UFH) or floor boarding preparation
• •**Second-fix electrics**: sockets; switches; lighting; consumer unit final connection; EV charger installation (if applicable)
• •**Second-fix plumbing**: radiators; taps; shower; bathroom fittings (client-supplied in RCB projects)
• •Kitchen installation (client-supplied units; contractor-fitted) or kitchen by a specialist kitchen installer
• •Internal joinery: skirting; architraves; doors; door furniture
• •Decorating: painter's preparation; primer; undercoat; finish coats
• •Floor finishes: tiles; engineered timber; vinyl (client-supplied in RCB projects)
• •External works: patio reinstatement; landscaping; garden restoration post-construction

*Note on second-fix materials (RCB standard)*: in RCB Design & Build projects, all second-fix materials (tiles; floor finishes; bathroom fittings; kitchen units; paint) are client-supplied. RCB installs; the client sources and supplies the materials. This is a standard RCB contractual position and should be confirmed in the specification before construction starts

*Timeline*: 4–8 weeks (depending on scope of finishes; client decision-making speed on second-fix material selections; lead times for kitchen delivery)

**Stage 12: Building Control completion and snagging (Construction Weeks 14–20)**:

• *What happens*:
• •Building Control final inspection: a Building Control officer inspects the completed extension to confirm compliance with all Parts. Inspections typically cover: structural completion; fire safety; means of escape; Part L energy compliance (including SAP/SBEM calculations for the extension); electrical installation certificate; drainage completion; ventilation compliance. The contractor must provide the following documents to Building Control at or before the final inspection: Electrical Installation Certificate (EIC) from the electrician (signed by a Part P competent person); Gas Safety Certificate (if a new gas appliance was installed); any specialist certification required (structural engineer's sign-off; fire door certification; acoustic test results if required)
• •Building Control issues the **Completion Certificate** — the legal document confirming that the extension complies with the Building Regulations. Keep the Completion Certificate permanently — it is required by mortgage lenders; insurers; and on any future sale of the property
• •Snagging: the contractor and homeowner walk through the completed extension together to identify any items that need correction — typically minor items (paint; alignment; silicone sealing). A snagging list is prepared and the contractor addresses all items before the final retention is released

*Timeline*: allow 2–4 weeks for final inspection and Completion Certificate issuance after practical completion of works

**Summary — total extension programme from idea to Completion Certificate**:

| Stage | Duration | Key activities | Key risks | |---|---|---|---| | Design concept + feasibility | 2–6 weeks | Measured survey; design options; feasibility check | Indecision on design direction delays subsequent stages | | Planning application | 8–16 weeks | LPA determination; surveys if needed | Refusal; conditions; LPA delays | | Technical design | 4–8 weeks | SE design; architectural production drawings | Structural complexity requiring redesign | | Building Regulations | 4–8 weeks | Full Plans approval | LPA queries; structural plan check | | Party Wall | 6–14 weeks | Notice service; award (if dissent) | Neighbour delay; complex award | | Procurement | 2–6 weeks | Tendering; contractor selection; contract | Re-tendering if pricing over budget | | Pre-construction | 2–6 weeks | BOA; utilities; CDM; long-lead orders | BOA delays; utility diversions | | Groundwork | 2–4 weeks | Foundation; drainage | Ground conditions; obstructions; weather | | Superstructure | 3–6 weeks | Walls; roof; structural opening | RSJ delivery; weather; structural issues | | First-fix | 2–4 weeks | M&E; insulation; airtightness | Trade availability; material delivery | | Second-fix + finishes | 4–8 weeks | Finishes; installations; decoration | Client material delays; trade sequencing | | Completion + snagging | 2–4 weeks | BC final inspection; snagging | BC officer availability; remedials | | **Total** | **24–50 weeks** | | |