Rear Extension London: Planning, Costs and What to Expect

The term 'rear extension' covers several different forms:

**Single-storey rear extension**: extends the footprint of the house rearward at ground floor level only. The most common form. Typically used to create a larger kitchen, kitchen-diner, or ground floor living room. A flat or pitched roof. Usually 3–6 metres deep.

**Double-storey rear extension**: extends two floors rearward — a ground floor extension with a first floor above. Adds both ground floor kitchen/living space and an upper floor bedroom or bathroom. Requires full planning permission (cannot be done under Permitted Development). More complex structurally — requires a full structural frame, party wall agreement, and more detailed Building Regulations application.

**Wraparound extension**: combines a rear extension with a side return infill, creating an L-shaped or fully wrapped ground floor footprint. The single most impactful ground floor project for a Victorian or Edwardian terraced house.

**Infill extension**: fills in an awkward corner or existing recess in the building's footprint. Often used where the rear of the house already has a Victorian addition and there is a recessed section to fill.

**Outbuilding / garden room**: separate from the main house rather than attached. A distinct category with different planning rules — generally more permissive under Permitted Development for outbuildings.

Most single-storey rear extensions in London can proceed without planning permission under Permitted Development (PD) rights, subject to limits.

• **Permitted Development limits for single-storey rear extensions (Class A, GPDO)**:
• •Terraced and semi-detached houses: up to 3 metres deep (or 6 metres under the Larger Home Extension Scheme)
• •Detached houses: up to 4 metres deep (or 8 metres under LHES)
• •Maximum height: 4 metres to ridge; 3 metres at the eaves
• •Must not extend beyond the principal (front) elevation
• •Materials must be similar in appearance to the existing house
• •The extension must not cover more than 50% of the total garden area

**The Larger Home Extension Scheme (Prior Approval)**: since 2019, extensions up to 6m (semi/terraced) or 8m (detached) can proceed under a Prior Approval process — not full planning permission, but a 42-day notification period during which neighbours can object. The council assesses only the impact on neighbours — not design or appearance.

• **When planning permission is always required**:
• •Double-storey extensions
• •Extensions exceeding PD limits
• •Properties in Conservation Areas where the extension affects the character
• •Listed buildings (Listed Building Consent required in addition)
• •Article 4 Direction areas where PD rights have been removed
• •Flats and maisonettes (PD rights do not apply)

**Lawful Development Certificate**: even where PD applies, we strongly recommend obtaining an LDC from the council (£206 application fee; 6–8 week determination) to formally document that the extension is lawful. Solicitors require this evidence when the property is sold.

In most London Victorian and Edwardian terraces, the rear wall of the main house is load-bearing. Creating an opening between the existing house and the new extension — to allow the two spaces to flow together — requires the careful removal of this wall and installation of a structural steel beam (RSJ).

This is the most critical structural element in any rear extension. Getting it wrong leads to settlement, sticking doors, cracking plasterwork, and in extreme cases structural movement. Getting it right is a matter of:

1. A structural engineer designing the steel beam, specifying the bearing length on each side, and designing the padstones 2. Correct temporary propping before any wall removal begins (acrow props and needle beams) 3. Correct bearing — the steel must bear on solid masonry for the designed bearing length (typically 150–200mm minimum each side) 4. Fire protection of the steel (intumescent paint or board encasement as per the Building Regulations specification)

• **Cost of the structural opening** (London, 2026):
• •2.5m–3.0m wide opening: £4,500–£7,500
• •3.5m–4.0m wide opening: £6,500–£10,000
• •Full-width opening (4.5m+): £9,000–£15,000

These costs include the structural engineer's design, temporary propping, steel supply and installation, padstones, and initial making good. They do not include plastering and final decoration.

The structural opening should always be itemised separately in any contractor's quote. A vague 'structural works' line does not give you confidence that the cost is realistic.

The foundation design for a rear extension in London depends on the subsoil conditions and the proximity of existing foundations and trees.

• **London clay**: most of London sits on London clay, which is highly shrinkable — it expands when wet and shrinks when dry. Trees draw moisture from clay, causing shrinkage and settlement. London clay requires:
• •Foundations deep enough to avoid the zone of seasonal moisture change (typically 1.5–2.5m in London, deeper where large trees are nearby)
• •Greater caution where trees (particularly willows, oaks, poplars, and limes) are within 10–15 metres of the new foundation
• •In some cases, a structural engineer will specify piled foundations (driven or bored piles) rather than strip foundations where tree proximity makes conventional depths impractical

**Proximity to existing foundations**: the new extension foundation must not undermine the existing house foundation. In Victorian terraced houses, the existing foundation is typically 600–900mm deep (much shallower than modern requirements). The structural engineer must account for this in the foundation design — typically by stepping the foundation or specifying the new foundation depth relative to the existing.

• **Costs**:
• •Standard strip foundation (1.5m–2.0m depth): approximately £150–£250 per linear metre
• •Deep strip foundation (2.0m–3.0m depth): £250–£400 per linear metre
• •Mini-piled foundation (tree proximity, difficult ground): £8,000–£20,000 for a typical extension footprint

Foundation design is one area where it is dangerous to cut costs. Structural movement from inadequate foundations is expensive and disruptive to correct — and it can take 3–5 years to manifest.

Rear extension costs in London vary considerably by size, height, specification, and location within Greater London.

**Single-storey rear extension cost ranges (London, 2026, excluding VAT)**:

| Size | Basic specification | Mid specification | Premium specification | |---|---|---|---| | 3m × 4m (12m²) | £42,000–£55,000 | £55,000–£72,000 | £70,000–£95,000 | | 4m × 5m (20m²) | £52,000–£68,000 | £68,000–£88,000 | £85,000–£115,000 | | 5m × 6m (30m²) | £65,000–£85,000 | £85,000–£110,000 | £105,000–£145,000 |

**What these ranges include**: structural works (foundation, superstructure, structural opening, roof), first-fix plumbing and electrics, plastering and boarding, windows and doors, and basic decoration. They do not include kitchen supply and fit, flooring, bifold/sliding doors (often quoted separately at £5,000–£18,000), or VAT.

• **What drives costs up**:
• •Deep foundations required by trees or difficult ground
• •Large structural openings (full-width steel)
• •Premium glazing (bifold doors, roof lantern, full-width glass roof)
• •High-specification finish level (limestone floors, bespoke joinery, underfloor heating throughout)
• •Conservation area: additional design requirements and sometimes required materials
• •Occupied property: additional care and containment versus vacant

**VAT**: standard rate (20%) applies to most rear extension works. Conversions of non-residential buildings to residential use may qualify for reduced or zero rate — not applicable to standard residential rear extensions.

A single-storey rear extension in London typically takes 12–18 weeks on site from start to practical completion.

• **Typical programme breakdown**:
• •Enable/demolition (strip existing lean-to, remove old extension if present): 1–2 weeks
• •Groundworks and foundations: 2–3 weeks (including concrete cure time)
• •Superstructure (walls, structural frame): 2–4 weeks
• •Roof structure and weatherproofing (EPDM, roof lantern, flashings): 1–2 weeks
• •Structural opening (temporarily prop, remove wall, install steel, make good): 1 week
• •External doors and windows: 1 week
• •First-fix plumbing, underfloor heating, electrics: 2–3 weeks
• •Screed (if wet UFH): 1 week cure
• •Plastering: 1–2 weeks
• •Second fix and decoration: 2–4 weeks

**Managing disruption**: most families can stay in the property throughout. The kitchen is disrupted during the structural opening phase and during kitchen installation. A good contractor will manage this disruption — temporary hoarding between old and new, dust control, and a clear communication plan.

**Building Control**: the contractor should manage all Building Control inspections as part of the project. You should receive a Building Control Completion Certificate at the end — this is important for future sale and remortgage. If your contractor does not proactively arrange this, ask directly.