Types of House Extension: Which Is Right for Your Property and Budget?

**1. Rear extension**:

*What it is*: An extension projecting from the rear elevation of the house into the back garden. The most common domestic extension type in London.

*What it achieves*: Adds floor area to the ground floor — typically used to extend a kitchen-dining room, create an open-plan kitchen-living space, add a utility room, or create a garden room or family room. The extension replaces a section of back garden, so the net benefit is measured against the loss of outdoor space.

*Sizes*: Single-storey rear extensions range from 3m to 8m deep (beyond 3m–4m the Prior Approval Neighbour Consultation Scheme or planning permission applies). Double-storey rear extensions project 3–6m deep and add a bedroom or bathroom above.

*Planning*: Single-storey rear extensions up to 4m (detached house) or 3m (semi-detached/terrace) are typically PD. Extensions 4–8m deep (detached) or 3–6m deep (semi/terrace) can use the Prior Approval Neighbour Consultation Scheme (45-day consultation). See permitted-development-rights-guide.

*Cost range (construction only, excluding client-supplied finishes)*: £35,000–£85,000 for a single-storey rear extension (4–6m × 5m footprint); £60,000–£120,000 for double-storey rear extension.

**2. Side return extension**:

*What it is*: An extension filling in the narrow side passage that runs along the side of many Victorian and Edwardian terrace and semi-detached London houses. The passage (typically 1–2.5m wide) runs from the front of the house to the back garden. The side return extension fills this passage and creates a wider ground floor space.

*What it achieves*: The side return is the magic trick of London house renovation — filling it in transforms the kitchen from a narrow galley into a much wider, light-filled space that can accommodate an island, a dining table, or seating. It does not add significant square footage (the passage is typically 10–25m² in plan) but dramatically changes the feel of the ground floor.

*Key design feature*: A side return extension works best with a full-height or near-full-height roof light (lantern or linear rooflight) running the length of the extension — this brings light into the ground floor from above, replacing the light that the passage previously admitted.

*Planning*: Side return extensions are typically PD provided they meet the Class A conditions (do not exceed the eaves height of the existing house, materials match, etc.). In Conservation Areas, side extensions may require planning permission if visible from the highway.

*Cost range*: £35,000–£60,000 for a side return extension combined with rear element (most side return extensions are built as an L-shaped extension combining a side return and a small rear element).

**3. Wrap-around extension**:

*What it is*: An L-shaped extension combining a side return element and a rear extension — the extension wraps around the rear corner of the house. This is the most impactful ground floor extension type for a terrace or semi.

*What it achieves*: Combines the benefits of both extension types — full width to the rear (occupying the side return passage as well as extending rearward) creates a very wide open-plan ground floor. The most common footprint is 4–6m rear projection × full house width plus the side return passage width.

*Planning*: A wrap-around extension is typically split between a PD rear element and a PD side element — but the combination may push the total added volume over the PD limits. Planning permission is commonly required for wrap-around extensions. See planning-permission-extensions-guide.

*Cost range*: £55,000–£100,000 for a single-storey wrap-around extension.

**4. Double-storey extension**:

*What it is*: An extension on two floors — a ground floor addition below and a first floor addition above, sharing a single set of new external walls.

*What it achieves*: Adds both living and bedroom/bathroom space in a single project — significantly more cost-effective per square metre than building two separate extensions at different times. Common configurations: ground floor kitchen extension + first floor bedroom; ground floor family room + first floor en-suite bathroom over an existing bedroom.

*Planning*: Double-storey rear extensions require planning permission in most cases (the PD volume limits are difficult to meet with two-storey construction). Double-storey side extensions almost always require planning permission.

*Cost range*: £65,000–£130,000 for a double-storey rear or side extension.

**5. Kitchen extension / orangery**:

*What it is*: A ground floor extension specifically designed as a kitchen or garden room space. Often incorporates large areas of glazing (bifold or sliding doors to the garden), a flat or pitched glazed roof (lantern or skylights), and a change of floor level from the existing house (step down to the extension).

*What it achieves*: The most 'showpiece' type of extension — strong visual impact, excellent garden connection, high natural light levels. Primarily adds floor area and lifestyle quality rather than bedroom accommodation.

*Cost range*: £45,000–£90,000 for a well-specified kitchen extension.

**6. Infill extension**:

*What it is*: An extension filling an existing recess or notch in the house footprint — a gap between a two-storey section and a single-storey section at the rear (common in Victorian terraces where the main body of the house projects further back on one side).

*What it achieves*: Fills in an inefficient recess, typically at first floor level, to create a new bedroom or bathroom. Often the most cost-effective extension type because it uses the existing side walls and only requires one new gable or dormer wall.

*Cost range*: £20,000–£40,000 for a small infill extension (typically 10–20m²).

**Decision matrix: what drives the choice of extension type**:

| Priority | Best extension type | |---|---| | Open-plan ground floor kitchen-dining | Rear extension or wrap-around | | Wider ground floor without losing garden | Side return extension | | Both living and bedroom space in one project | Double-storey rear extension | | Maximum garden connection and light | Kitchen extension with large glazing | | Bedroom only, minimal garden loss | Loft conversion (not an extension) | | First floor bedroom without planning permission | Infill extension (often PD) | | Best cost per m² of new space | Double-storey (amortises foundations/roof cost over more floor area) |

**Property-type suitability**:

• *Victorian/Edwardian terrace (2-storey)*:
• •Side return: ✓ Excellent (side passage typical of Victorian terraces)
• •Rear single-storey: ✓ Excellent (standard project)
• •Wrap-around: ✓ Very good (most impactful, often requires planning)
• •Double-storey rear: ✓ Good (most require planning; strong ROI)
• •Loft conversion: ✓ Excellent alternative to extension for bedroom

• *1930s semi-detached*:
• •Rear single-storey: ✓ Excellent
• •Double-storey rear: ✓ Good
• •Side extension: ✓ Possible but affects visual relationship with detached neighbour
• •Hip-to-gable loft: ✓ Excellent (large volume potential)
• •Side return: Generally not applicable (1930s semis typically have a wider side passage or no passage)

• *Detached house*:
• •All extension types possible depending on site
• •Double-storey side extensions often visually appropriate (can balance an asymmetric original house)
• •Rear extension footprint not limited by adjacency to neighbours

**Budget-based decision**:

• For homeowners with a fixed budget, the priority should be:
• •Under £40,000 construction: side return extension or infill; not a rear extension
• •£40,000–£70,000 construction: single-storey rear extension (standard size); or side return + small rear element
• •£70,000–£100,000 construction: wrap-around or modest double-storey
• •£100,000+ construction: double-storey or large single-storey rear

**Planning-based decision**:

• For homeowners who want to avoid a planning application:
• •Prioritise single-storey rear extensions within PD limits (4m detached, 3m semi/terrace; or up to 6m/8m under Prior Approval Scheme)
• •Velux-only loft conversions within Class C PD limits
• •Small infill extensions within volume limits
• •Larger extensions that require planning permission are worth pursuing — most straightforward rear extensions in non-Conservation Areas are approved — but the planning application adds 3–4 months and £258+ to the process

**Structural implications by extension type**:

*Rear extension (single-storey)*: Ground floor bearing walls are retained; the extension has its own foundations; the junction between the new extension roof and the existing house rear wall requires careful detailing (weathering, structural connection). Where the rear wall of the house is load-bearing (which it typically is in Victorian terraces), a structural opening must be made through the rear wall — typically an RSJ steel beam — to create the open plan. This structural opening is a Building Regulations matter (Part A) requiring structural engineer specification.

*Side return extension*: The side wall of the house (the party wall or the side wall of a detached house) is typically retained; the extension is built against it. The new roof structure spans between the retained side wall and the new front wall of the extension. A full-height rooflight typically runs the length of the extension between the retained side wall and the new extension front wall.

*Wrap-around extension*: The most complex structural arrangement — the rear and side elements of the extension must be coordinated, including the corner post or pier at the junction. The structural opening through the rear wall to create the open plan is the most significant structural element.

*Double-storey extension*: Requires more significant foundations than a single-storey (greater imposed load); the first floor and staircase structure must be designed by a structural engineer. The junction between the new second floor and the existing first floor must be carefully detailed.

**M&E implications by extension type**:

• All extension types require:
• •New electrical circuits (ring main for sockets, lighting circuit)
• •Heating extension (radiators, or underfloor heating in the extension slab, connected to the existing boiler)
• •Possible boiler upgrade if the existing boiler does not have sufficient capacity for the additional heated floor area

• Extensions involving a new kitchen also require:
• •New drainage connection for sink
• •Gas supply to new hob position (if gas)
• •New extraction duct to external wall

• Extensions involving a new WC, utility room, or bathroom also require:
• •New drainage connection to the existing soil stack
• •Additional hot water demand (if the extension includes a shower or bath)