Rear Extensions in London: Planning, Design, and Cost Guide

**Single-storey rear extensions — when planning permission is needed vs. not**:

Under the General Permitted Development Order (GPDO) 2015 Schedule 2 Part 1 Class A, single-storey rear extensions are permitted development (PD) — i.e., no planning permission is required — subject to meeting all conditions:

• *PD conditions for single-storey rear extensions*:
• •*Maximum depth*: In a terraced or semi-detached house, 3.0m from the original rear wall of the house; in a detached house, 4.0m from the original rear wall. (The 'original' rear wall means as built, not as extended by any previous extension.)
• •*Expanded depths under Prior Approval (Neighbour Consultation Scheme)*: Up to 6.0m (terraced/semi) or 8.0m (detached) is permitted under the Neighbour Consultation Scheme (sometimes called 'Prior Approval') — a fast-track planning process involving notification to neighbours and a 21-day consultation period. This scheme has been made permanent.
• •*Maximum height*: Single-storey rear extension — maximum 4.0m height (to ridge or flat roof); eaves height not to exceed 3.0m if the extension is within 2.0m of the boundary.
• •*Not in front of the principal elevation*: The extension must be entirely behind the front wall of the house.
• •*External materials*: Must be similar in appearance to the existing house (bricks, tiles, render).
• •*No covered verandas, raised platforms, or balconies*.
• •*Not in a Conservation Area* (Conservation Areas require planning permission for all rear extensions where the extension is visible from the highway, which for a rear extension in a typical London terrace, it may not be — but Conservation Area status should be checked).

*The 'original rear wall' rule*: The depth is measured from the original rear wall of the house as it was built (or as at 1 July 1948 for older buildings). A previous extension has already used part of the PD allowance. A 2.0m extension built by a previous owner means the remaining PD allowance is reduced by 2.0m (so a terraced house has 1.0m remaining for PD, and only 4.0m remaining under the Prior Approval scheme).

**Double-storey rear extensions — planning permission always required**:

• Double-storey rear extensions never qualify as PD — planning permission is always required. A double-storey rear extension is one that extends above the ground floor ceiling level. The LPA's key assessment criteria:
• •*Depth*: LPAs are reluctant to permit double-storey rear extensions that are deeper than the single-storey PD allowance (typically 3.0m–4.5m for approved double-storey extensions in most London boroughs)
• •*Overlooking*: First-floor windows must not cause unacceptable overlooking of neighbours' gardens — engineers typically specify obscure glazing or high sills on side-facing first-floor windows in the extension
• •*Distance from rear boundary*: The extension must be at least 7.0m from the rear boundary (for a double-storey extension) in most London boroughs
• •*Materials*: must match the existing house in appearance
• •*Terracing effect*: the LPA will assess whether the double-storey element creates an unacceptable visual terracing or dominance effect

**Conservation Areas and Article 4 Directions**:

• •In a Conservation Area, the rear extension requires planning permission if it is visible from any highway — for a typical rear extension in a London terrace, this is usually not the case (the rear garden is not visible from the street), so some rear extensions in Conservation Areas remain PD. However, check with the LPA.
• •In areas with Article 4 Directions removing Part 1 PD rights (Islington, Hackney, Camden, parts of Lambeth), all extensions require planning permission regardless of size.

**Foundations**:

Foundation design for a rear extension depends heavily on: 1. The soil type (London Clay in most of inner London, river terrace gravels in Thames-side locations, made ground in former industrial areas) 2. The presence and proximity of trees — especially significant on London Clay (see building-near-trees-guide) 3. The existing foundation type and depth (which the new extension foundation must not undercut or disturb)

• Typical foundation types and depths for a rear extension in London:
• •No trees within 15m, non-clay or post-1945 property: 900mm trench fill (standard)
• •London Clay, moderate tree influence (15m distance from a medium tree): 1.5m–2.0m trench fill or wide strip
• •London Clay, significant tree influence or large trees: 2.0m–3.5m trench fill or mini-piles
• •Where trees force depths over 3.5m: CFA or rotary bored mini-piles

**Party Wall Act implications**:

• A rear extension in a terraced or semi-detached house will almost always trigger Party Wall Act obligations:
• •Section 6 notice: required where excavation for foundations is within 3.0m of a neighbour's building AND the new foundation extends to a depth lower than the neighbour's existing foundation (applicable to most London rear extensions with new foundations)
• •Section 2 notice: required for any works to the shared party wall (e.g., making good the connection of the new extension roof to the party wall)
• •Party wall notices must be served at least 2 months (Section 2) or 1 month (Section 6) before work starts
• •Neighbour can consent (building owner proceeds) or dissent (a party wall award is required, involving a party wall surveyor or agreed surveyor)

**Structural opening (RSJ) to connect to the existing house**:

• Creating the open plan connection between the existing rear reception room and the new extension almost always involves:
• •Removing the existing rear wall (or a significant portion of it) — which in a Victorian terrace is structural, carrying the floors and roof above
• •Installing a steel beam (RSJ/Universal Beam, sized by structural engineer) to span the opening
• •Installing padstones at each end of the beam
• •Temporary propping during the wall removal (see open-plan-living-structural-guide)

This steel is often the most significant single structural cost in the extension beyond the foundations.

**Roof design — flat vs. pitched extension roof**:

• *Flat roof (the most common for London rear extensions)*:
• •Cold deck or, more commonly, warm deck construction
• •Warm deck: 150–200mm PIR insulation laid directly on the structural deck (OSB or concrete), with single-ply waterproofing (Sarnafil, Firestone) or GRP on top
• •Access for maintenance: a rooflights surround or hatched access point should be designed in
• •Rooflights: flat roof extensions almost always have one or more rooflights (VELUX, Korniche, or roof lantern) to bring daylight into the centre of the plan away from the rear elevation glazing
• •Upstand height: waterproofing upstands at the junction with the existing wall must be at least 150mm above the finished flat roof level

• *Lean-to or pitched roof (less common, typically matches the host house)*:
• •Typically single-pitch (mono-pitch) leaning against the existing rear wall
• •Can match the host house materials (concrete or clay tiles) — often required in Conservation Areas
• •Greater ridge height than a flat roof extension of the same depth — relevant to the ADK height limit

**External glazing — bifold doors, sliding doors, and rooflights**:

• The rear elevation of the extension typically features large glazed openings to connect the interior to the garden:
• •Bifold doors: open up to 90% of the rear elevation width; popular in 3.5m–6.0m wide openings; cost £2,500–£6,000 for uPVC, £5,000–£14,000 for aluminium 3-panel to 5-panel
• •Sliding pocket/lift-and-slide doors: very wide openings up to 6m+; smoother operation; cost £4,000–£12,000+
• •Fixed glazing + opening casements: lower cost, lower thermal performance at the largest sizes

For Part L compliance, all new windows and doors must achieve a minimum U-value of ≤ 1.4 W/m²K (double glazed minimum).

**Rear extension cost ranges (London, 2025)**:

Costs are based on a standard rear extension to a typical Victorian/Edwardian terraced house. All costs are for contractor supply and installation unless noted as client-supplied.

• *Single-storey rear extension (3.0m–4.0m depth, full width ~3.5m = 10.5m–14m²)*:
• •Budget specification (standard brick, flat roof, UPVC bifolds, basic finishes): £38,000–£52,000
• •Mid-range specification (stock brick to match, warm roof, aluminium bifolds, quality fit-out): £52,000–£70,000
• •Premium specification (bespoke joinery, structural glazed roof, polished concrete floor): £70,000–£100,000+

*Single-storey rear extension (5.0m–6.0m depth or wrap-around): see wrap-around-extension-guide for larger schemes.*

• *Double-storey rear extension (3.0m depth, adds two rooms)*:
• •Budget to mid-range: £75,000–£110,000
• •Mid-range to premium: £110,000–£155,000+

• **What's typically included in the quoted price**:
• •Strip and excavate for foundations
• •Foundations (trench fill, standard depth — deeper foundation premium applies for tree influence)
• •Drainage connections (connection of extension floor drainage to existing system)
• •Structural blockwork or timber frame walls
• •Roof structure and waterproofing (single-ply or GRP flat roof)
• •Insulation to Part L
• •Windows and external doors (bifold or sliding — specified quality)
• •Structural steel (RSJ) to open existing rear wall
• •First and second fix electrical (lighting, sockets, smoke detection)
• •First and second fix plumbing to the point of kitchen/WC fit-out
• •Plastering and making good
• •Structural engineer's fees
• •Building Regulations application and inspection fees

• **What's NOT typically included (client-supplied or separate contract)**:
• •Planning fees (£258 for householder application) and planning consultant fees
• •Architect fees (£1,500–£4,000 for planning drawings; £1,500–£3,500 for building control drawings)
• •Party wall surveyor fees (£600–£1,500 per surveyor if a dispute arises)
• •Kitchen supply (allowance only)
• •Second-fix materials: floor tiles, worktops, sanitaryware — client supplied per RCB standard conditions
• •External landscaping / patio reinstatement
• •Carpet

• **Programme — how long does a rear extension take**:
• •Planning application (if required): 8–13 weeks from submission
• •Pre-start (party wall, structural design, materials procurement): 4–8 weeks after planning
• •On site — single-storey extension: 10–16 weeks from groundworks start to practical completion
• •On site — double-storey extension: 18–28 weeks

*Note: the single largest programme risk is the party wall process — if a neighbour dissents and a party wall award is required, this can add 2–6 weeks before site start is possible.*