Extending a Terraced House in London — Options, Costs, and What to Expect

A mid-terraced house (attached on both sides) has fewer extension options than a detached or semi-detached, but the options that exist can be transformative:

**1. Rear extension**: the most common, extending the footprint to the rear into the garden. Can be single-storey (most common) or double-storey. Single-storey rear extensions are usually Permitted Development within depth limits.

**2. Side return extension**: the narrow passage alongside the house (the 'side return' — common on Victorian terraces) is enclosed to add width to the ground floor. A standalone side return extension adds 1.5–3 metres of width across the whole rear part of the ground floor — typically turning a narrow kitchen into a full-width kitchen-dining space.

**3. Wrap-around extension**: combines a rear extension and a side return infill — an L-shape that maximises ground floor footprint. The most popular choice for families who want a genuine open-plan ground floor.

**4. Double-storey extension**: extends two floors at the rear, adding a room on the ground floor and one or more bedrooms above. Always requires planning permission. More expensive, but adds more space.

**5. Loft conversion**: not a ground-floor extension, but often done in combination with one. Adds bedrooms at the top of the house.

**End-of-terrace note**: an end-of-terrace (attached on one side only) has additional side extension options — a true side extension, not just a side return. This significantly increases the potential footprint.

Extension costs for London terraced houses (2025):

• **Single-storey rear extension:**
• •3m × 4m: £40,000–£60,000
• •4m × 5m: £50,000–£75,000
• •5m × 6m: £65,000–£90,000

• **Side return extension (standalone):**
• •Typical London side return (2m wide × 5m long): £30,000–£50,000

• **Wrap-around extension (rear + side return):**
• •Standard specification: £65,000–£90,000
• •With glazed lantern and bifolds: £85,000–£120,000

• **Double-storey rear extension:**
• •3m × 4m (two floors): £70,000–£100,000
• •4m × 5m (two floors): £90,000–£130,000

• **Loft conversion (dormer):**
• •One bedroom + en-suite: £45,000–£70,000
• •L-shaped (two bedrooms): £55,000–£85,000

**Combined projects** (extension + loft) are common in London terraces. Running them as one project saves on scaffold costs, disruption, and preliminary costs — typically 10–20% cheaper than running them separately.

All costs above include build, structural works, glazed door and window installation, and Making good — but exclude VAT, kitchen and bathroom units (client-supplied), specialist finishes, and furniture.

Planning rules for terraced houses are specific in several ways:

• **Single-storey rear extension:**
• •Permitted Development (no planning needed): up to 4 metres deep from the original rear wall
• •Prior Approval (Neighbour Consultation Scheme): 4–6 metres deep
• •Over 6 metres: full planning permission required

• **Side return extension:**
• •May be Permitted Development if no wider than half the original house width, single-storey only
• •In Conservation Areas, side extensions visible from the street typically require planning permission

• **Double-storey rear extension:**
• •Always requires full planning permission
• •No Permitted Development route for two-storey rear extensions

• **Loft conversion:**
• •Rear dormer typically Permitted Development (conditions apply)
• •Front dormers and hip-to-gable conversions typically require planning permission

• **Conservation Areas (very common in inner London)**:
• •Rear extensions: may still be PD if not visible from the street
• •Side extensions and any extension visible from the highway: planning permission required
• •External materials, windows, and doors: must be appropriate to the area

**Article 4 Directions**: some London boroughs and specific streets have removed PD rights. This is more common than many people think — Islington, Hackney, Kensington & Chelsea, Camden, and others have extensive Article 4 coverage. Always check before assuming PD applies.

The right extension depends on what you want to achieve:

**If your primary goal is open-plan kitchen-dining-living**: A wrap-around extension (rear + side return) or a large single-storey rear extension. The wrap-around is better if you want width (the side return gives you 1.5–3 metres of extra width). A pure rear extension is better if you want more depth.

**If your primary goal is adding a bedroom**: A loft conversion is almost always more cost-efficient than a double-storey rear extension for adding one bedroom. A double-storey extension makes more sense if you also want to add significant ground floor space.

**If your primary goal is value**: In London, the hierarchy of value uplift by project type is approximately: 1. Loft conversion (adding a bedroom, especially bedroom 3 or 4) 2. Wrap-around extension (open-plan ground floor) 3. Single-storey rear extension (larger kitchen-dining) 4. Double-storey extension (adds most space but costs most)

**If budget is constrained**: A side return extension is often overlooked but is one of the better-value projects for a London terrace — it adds significant functional width to the kitchen-dining area at a relatively modest cost (£30,000–£50,000).

**If you want to maximise all potential**: Run a loft conversion, a wrap-around extension, and reconfigure the ground floor all in one project. This is a major undertaking (6–9 months on site, £150,000–£280,000+) but transforms a narrow 3-bedroom terraced house into a 4–5 bedroom, generous, open-plan family home.

Before work starts, most London terraced house extension projects require:

☑ **Design drawings**: architectural or design-and-build drawings showing the proposed works

☑ **Planning or LDC**: Permitted Development Lawful Development Certificate, or full planning permission, depending on the route

☑ **Party Wall Notice**: served on both neighbours (both sides of a mid-terrace) at least 2 months before work starts. For rear extensions, the party wall obligation affects the side-wall neighbour; for excavations within 3m, it affects the rear neighbour too in some cases

☑ **Structural engineer**: structural calculations and drawings for the new floor/roof/beams, submitted to Building Control

☑ **Building Regulations application**: Full Plans or Building Notice submitted before work starts

☑ **Contractor appointment**: programme, scope, and price agreed and contract signed

☑ **Kitchen/bathroom specification**: if including a kitchen or bathroom, these decisions must be made before first-fix begins — not partway through the build

RCB manages items 1 through 6 as part of our pre-construction service, coordinating design, planning, party wall, and structural engineering in a structured process before any ground is broken.