Extending a Semi-Detached House in London: Planning, Design and Costs

**Side extension:** A single-storey side extension on the side elevation of a semi-detached house uses the space between the house and the side boundary — where there is typically 2–4 metres of space (compared with the 0.9–1.8m passage in a Victorian terrace). This allows a full-width side extension that can create a large utility room, WC, or additional reception space.

Planning: a single-storey side extension on a semi-detached house may be permitted development if it does not exceed half the width of the original house. However, in many London boroughs, Article 4 Directions restrict or remove PD rights for side extensions on semis, on the grounds of preserving the 'gap' character between properties. Always check.

Two-storey side extensions always require planning permission.

**Hip-to-gable loft conversion:** Semi-detached houses typically have a hipped roof — the roof slopes on the side elevation as well as the rear. Converting the hip to a gable (a flat vertical end wall) dramatically increases the usable loft floor area. On a standard inter-war semi, a hip-to-gable conversion can add 25–40% more usable loft floor area compared with a rear dormer on the same property.

Planning: hip-to-gable conversions on semi-detached houses require planning permission — they affect the side elevation. Conservation Area properties require permission regardless of type. On detached houses, hip-to-gable can sometimes be PD, but not on semis in most London boroughs.

Cost: hip-to-gable loft conversion with rear dormer and en-suite (building works): £55,000–£80,000.

**L-shaped dormer:** In a detached or semi-detached house with a rear addition, the L-shaped dormer — which extends over both the main roof and the rear addition roof — maximises the loft conversion footprint. Often the most floor area achievable in a loft conversion on a semi.

**Wraparound extension:** A combination rear extension plus side extension, creating an L-shape or C-shape plan at ground floor. On a semi, the side element can be wider than on a terrace (no party wall constraint on the side), potentially creating a very large new kitchen or family room footprint.

Wraparound extensions on semi-detached houses usually require planning permission (the side component exceeds PD limits or is on the side elevation).

• **Side extensions: PD vs. planning permission:**
• Permitted Development Class A allows single-storey side extensions on semis provided:
• •The extension does not exceed half the width of the original house
• •It is a single storey (maximum 4m height at the eaves)
• •It does not extend beyond the principal elevation

In practice, many London boroughs have Article 4 Directions that remove PD rights for side extensions on semi-detached houses. The most common reason given is the preservation of the 'gap' between properties — the rhythm of semi-detached pairs with a visible gap between them is considered a character feature of inter-war residential estates.

• **Hip-to-gable conversions:**
• Always require planning permission for semi-detached houses. The change to the side elevation (from sloped hip to vertical gable) is clearly visible from the street and adjacent properties. Planning officers assess:
• •Whether the new gable relates to the character of the building and the street
• •The impact on the paired semi (the other half of the semi-detached pair)
• •Materials and finishes

Most hip-to-gable conversions are approved on inter-war semis in London — it is a well-established and common conversion type. However, refusals do occur in Conservation Areas and where the proposal is considered too prominent.

**Two-storey side extensions:** Always require planning permission. These are assessed carefully as they can significantly affect the scale and character of the semi-detached pair and the street scene. The side elevation of a two-storey extension must be set back from the front elevation and must not appear to 'out-mass' the original house when viewed from the street.

**Rear extensions:** Same rules as a terrace — PD up to 3m depth (single-storey) without Prior Approval; up to 6m with Prior Approval (Larger Home Extension Scheme).

• **Hip-to-gable loft conversion (with rear dormer):**
• •1 bedroom, no bathroom: £45,000–£65,000
• •1 bedroom + en-suite (building works): £55,000–£80,000
• •2 bedrooms + bathrooms (L-shaped or hip-to-gable + rear dormer): £65,000–£95,000

• **Single-storey side extension (2.5m–4m wide × 4m deep, brick/block, flat roof):**
• •Basic (utility/WC): £30,000–£50,000
• •Mid-spec (kitchen-diner side extension): £45,000–£70,000

• **Two-storey side extension:**
• •£70,000–£120,000 (2 new rooms — one at ground, one at first floor)

• **Single-storey rear extension:**
• •Same ranges as for a Victorian terrace — typically £55,000–£95,000 for a standard 4m × 5m mid-spec
• •Semis often have a larger rear footprint than terraces, allowing a wider full-width rear extension

• **Wraparound (rear + side):**
• •£80,000–£145,000 (semi-detached wraparound is often larger than a terrace wraparound)

• **Party wall:**
• •A hip-to-gable conversion that builds on or close to the shared roof line may trigger the Party Wall Act (Section 2 — works to a party structure). Serve notice on the attached semi-detached neighbour and budget £1,500–£3,500 for party wall surveyor costs if they dissent.

The semi-detached house in London's outer zones (Zone 3–5) is typically the best extension value in the city. Here is why:

**Larger plots:** Inter-war London semis typically have larger rear gardens (15–20m+) and significant side space compared with Victorian terraces. This means rear extensions can go deeper (more floor area), and the side space opens up options not available to a terraced house.

**Hip-to-gable uniqueness:** The hip-to-gable conversion is only possible on a semi-detached or detached house — not a terrace. It provides significantly more loft floor area than a rear dormer on the same footprint, at only a modest additional cost.

**Bedroom count uplift:** A semi-detached house that starts as a 3-bedroom property can realistically become a 5-bedroom property (adding a loft conversion + side extension bedroom) at a fraction of the cost of moving. In Zone 3–5 London, the price premium between a 3-bed and a 5-bed semi in the same road can be £100,000–£250,000.

**Combined project approach:** The most transformative approach for a London semi is to combine: 1. A rear or wraparound extension (enlarged ground floor kitchen-diner) 2. A hip-to-gable loft conversion (new bedroom + en-suite) 3. An open-plan reconfiguration of the existing ground floor

All three in a single project, managed by one principal contractor, delivers the maximum transformation at lower combined cost than three separate projects.