Rear Extension Cost London 2025: What a Single-Storey Extension Really Costs

**The eight factors that most affect rear extension cost**:

**1. Size (m²)**: Extension cost is broadly linear with floor area. A simple single-storey rear extension costs approximately £1,800–£2,800/m² of new floor area in London (2025 mid-spec, excluding VAT and professional fees). At 20m², that is £36,000–£56,000. At 30m², £54,000–£84,000. Size is the primary cost driver — the price per square metre stays roughly constant; the total cost scales with area.

**2. Structural complexity**: Removing the existing rear wall and spanning the full width of the house with a structural steel beam (RSJ) is standard for an open-plan kitchen extension. The beam size, method of installation, and temporary support required add cost — typically £2,000–£5,000 for the structural beam, propping, and making good, plus the structural engineer's fees (£800–£1,500). Where existing wall removal is not required (the extension opens from a single door), structural cost is lower.

**3. Roof type**: Flat roof (warm roof construction: PIR insulation + single-ply membrane, 25-year guarantee): lower cost. Pitched roof (tiled or slated, with all associated roof structure): 15–25% more expensive. Glazed roof (lantern, rooflight, or full frameless glass roof): significantly more expensive — full-width frameless glass roofs for a 6m × 4m extension can add £12,000–£30,000+ over a flat roof. Roof choice significantly affects cost.

**4. Type and specification of glazing**: Standard double-glazed rear door + window: £1,500–£3,000. Aluminium bifold doors (3–4 panel): £3,500–£7,000 supply and fit. Aluminium sliding doors (large 4m+ opening): £4,000–£9,000. Full-width frameless structural glass: £15,000–£35,000. Glazing is one of the highest single-item variables in extension costs.

**5. Internal fit-out specification**: An extension that includes a full kitchen installation (fitted kitchen, appliances, kitchen electrical circuits, plumbing for sink and dishwasher, extraction) costs £8,000–£20,000 more than an extension that delivers a shell for a separately supplied kitchen. Whether the kitchen is included in the extension scope, and at what specification level, is critical when comparing quotes.

**6. External ground levels and drainage**: London rear gardens are often at different levels to the ground floor — requiring steps, ramps, or retaining walls within the extension scope. Drainage connections (new soil run for a kitchen sink, rainwater disposal from the flat roof) vary in complexity with site layout and distance to the existing drain. On some London sites, a Thames Water build-over agreement is required for foundations near a public sewer — which adds survey cost (£150–£300) and potentially structural constraints.

**7. Party wall and foundations**: For an extension within 3 metres of the neighbour's structure, a Section 6 party wall notice is required. Where the neighbour dissents, party wall surveyor costs of £600–£3,500 are added (the building owner pays). Foundation type is determined by the structural engineer based on ground conditions — most London extensions use trench-fill foundations at 1.0–1.2m depth; where shrinkable clay is present deeper down, mini-piles or engineered foundation solutions may be needed at additional cost.

**8. External works**: Paving, landscaping, and making good the garden after the extension is built are often excluded from extension quotes and can add £2,000–£10,000 depending on the garden area affected.

**RCB cost ranges for rear extensions in London (2025)**:

All figures are for labour and materials for the extension works only, excluding professional fees (architect, SE, Building Control), party wall costs, kitchen supply (client-supplied per RCB standard terms), floor finishes (client-supplied), and external works/landscaping.

**Standard single-storey rear extension — open plan kitchen/dining (20–25m²)**:

*What's included*: Substructure (foundations, oversite, DPC, insulated ground floor slab); blockwork cavity walls to eaves; flat roof (PIR warm roof + single-ply membrane); bi-fold or sliding aluminium rear doors (1 set); rooflight (1 × standard Velux or flat roof light); structural steel beam spanning full width; first-fix plumbing (kitchen supply and waste connections); first-fix electrical (ring main, lighting circuits, extraction); insulation to Part L (walls, floor, roof U-values compliant); plastering and skim; decoration of new walls and ceiling; making good to internal junction with existing house. *What's NOT included*: Kitchen units, worktops, appliances, sink (client-supplied); floor finish (client-supplied — tile, LVT, or hardwood); second glazing set beyond the standard allowance; external paving or landscaping; architect/SE/Building Control fees; party wall costs.

| Size | Cost Range | |---|---| | 18–20m², flat roof, 1 set bifolds, standard rooflight | £42,000–£58,000 | | 22–25m², flat roof, 1 set bifolds, larger rooflight | £52,000–£72,000 | | 28–32m², flat roof, bifolds + fixed glazing, lantern | £65,000–£90,000 |

**Single-storey side return extension (infilling the side return, 8–15m²)**:

*What's included*: Foundation, cavity wall (or glazed wall/roof to the side), structural works to remove existing kitchen wall into side return, new roof structure, glazing (glass or standard roof — depending on design), first-fix M&E, plastering, decoration.

| Size | Cost Range | |---|---| | 8–10m² side return infill, flat/glass roof | £28,000–£40,000 | | 12–15m² side return, full glass roof, aluminium structure | £38,000–£55,000 |

**Wrap-around extension (rear + side return combined)**:

| Size | Cost Range | |---|---| | 25–35m² wrap-around (rear + side), flat roof | £60,000–£90,000 | | 35–45m² wrap-around (rear + side), architectural glazing | £80,000–£120,000 |

**Double-storey rear extension**:

*Note*: Double-storey extensions always require planning permission (not permitted development). They involve significantly greater structural complexity (first-floor structure spanning new extension; stairs; new rooms at first floor) and scaffold requirements.

| Size | Cost Range | |---|---| | Double-storey, 20m² footprint (20m² per floor = 40m² total) | £90,000–£130,000 | | Double-storey, 25m² footprint (50m² total) | £110,000–£155,000 |

• **Professional fees (additional to above costs)**:
• •Architect (planning drawings + Building Control drawings): £2,500–£6,000
• •Structural engineer: £800–£2,000
• •Building Control Full Plans (LA or Approved Inspector): £400–£1,000
• •Party wall surveyor (if triggered): £600–£3,500
• *Total professional fees*: £4,300–£12,500 for a standard extension

**Why rear extension quotes vary so much**:

When homeowners get three quotes for the same rear extension and find a £20,000 spread between the lowest and highest, the difference is almost never contractor margin. It is almost always:

• •*Different scope assumptions*: One quote includes the kitchen fit-out; another doesn't. One includes the structural steel; another has it as a provisional sum. One includes 2 rooflights; another's price is for one.
• •*Different specification levels*: One contractor has priced bifold doors in aluminium; another in uPVC; a third has priced a standard hinged door.
• •*Different foundation assumptions*: One contractor has assumed standard 1m trench-fill foundations; another has allowed for deeper foundations or pilings based on their experience of similar sites nearby.
• •*Different contingency levels*: Experienced contractors build in margin for unforeseen conditions; less experienced contractors do not, making their quote look cheaper but ultimately leading to more variations and a higher final account.
• •*External works*: Some quotes include garden reinstatement; others explicitly exclude it.

**How to compare quotes accurately**:

To make a meaningful comparison, all three quotes must be based on the same scope document — the same drawings, the same specification, the same inclusions/exclusions list, and the same tender enquiry questions. If you ask three contractors to quote from a brief description in a WhatsApp message, you will get three quotes that are incomparable. See how-to-get-multiple-quotes-guide for the full tender process.

**Specific items to clarify in every rear extension quote**:

1. Does the price include the structural steel beam and installation? What size beam is assumed? 2. What is the assumed foundation type and depth? Has this been checked against a structural engineer's specification? 3. What glazing is included — size, type (bifold/sliding/fixed), material (aluminium/uPVC), specification? Is triple glazing allowed for? 4. Is the kitchen first-fix plumbing and electrical included? What circuits and positions are allowed for? 5. Does the price include a rooflight? What size? Is it a standard Velux, a flat-roof fixed light, or an opening light? 6. Is external paving/landscaping included? 7. What is excluded and listed as provisional sum? 8. Does the price include the Building Control application fee and all inspection visits to completion certificate? 9. What is the payment schedule — are stage payments linked to progress milestones?

**Red flags in rear extension quotes**:

• •A quote that is 30%+ below the other two with no detailed explanation of where the saving comes from
• •A quote with significant provisional sums (items priced 'to be confirmed' rather than fixed): structural works, steels, and foundations should be fixed, not provisional
• •A quote that does not specify what glazing is included
• •A quote that doesn't mention Building Control — who makes the application and who pays the fees