Extending a Detached House in London: More Options, Different Rules

The permitted development rules for detached houses are meaningfully more generous than for terraced and semi-detached:

• **Rear extension (single-storey):**
• •**Up to 4 metres** projection from the rear wall — without planning permission
• •**Up to 8 metres** under the larger home extension Prior Approval scheme

Compared with 3m (PD) / 6m (Prior Approval) for terraced and semi-detached — a significant difference that allows for much deeper rear extensions.

• **Rear extension (double-storey):**
• •Up to 3 metres from the rear wall
• •No closer than 7 metres to the rear boundary
• •Must not exceed eaves height of the original house
• •Requires planning permission in most other cases

• **Side extension (single-storey):**
• •Up to half the width of the original house
• •Single storey
• •Not on a designated land (conservation area, AONB, National Park)

• **Maximum height:**
• •Single-storey extension: maximum 4 metres (or same height as eaves of original house if lower)
• •Extension with dual-pitch roof: maximum ridge height equal to original house

**Outbuildings:** Detached houses with larger gardens can also make more use of outbuildings under permitted development — garden offices, studios, garden rooms up to 4 metres (dual-pitch) or 3 metres (flat/mono-pitch) and within the 50% curtilage rule. Outbuildings further than 2 metres from the boundary can be up to 4 metres, those within 2 metres are limited to 2.5 metres eaves height.

**Wraparound double extension:** With no shared walls on either side, a detached house can be extended on both sides simultaneously — creating a substantial increase in ground floor area. A detached house with 5 metres of space to each side could accommodate two side extensions, a rear extension, and wrap these elements together into a major replan of the ground floor.

**Detached garage conversion or garage replacement:** A detached garage or outbuilding can be converted to habitable use, or demolished and replaced with a larger structure. Converting an existing outbuilding (garage, workshop) to habitable use typically does not require planning permission where no material change of use is involved and the structure already exists, though Building Regulations will apply.

Replacing a detached garage with a new detached living space (annex, home office, garage replacement with room above) is a popular project on larger London detached plots — subject to the outbuilding size limits and curtilage rules.

**Substantial rear extension with pool or leisure room:** Larger detached plots sometimes accommodate substantial ground floor extensions with specialist uses — home gym, pool building, cinema room. These uses are typically acceptable as ancillary to the main dwelling but may require planning permission for certain structures or where the scale of the addition is very large.

**Double-storey side and rear:** A full double-storey extension on a detached house — side and rear, or rear only — can be a major addition that transforms the property. A detached house of 100m² can potentially be extended to 200m²+ with a comprehensive double-storey scheme.

**Party wall — generally simpler for detached:** A detached house has no shared walls with neighbours — which means Section 2 party wall obligations (works to a party wall) generally do not arise. However, Section 6 (excavation near neighbouring foundations) can still arise where new foundations are within 3 metres of a neighbouring building and are deeper than their foundations. Check with a party wall surveyor before assuming no party wall obligations on a detached extension.

**Planning considerations for detached extensions:** Where planning permission is required for a detached house extension:

• •**Size and scale:** The LPA will assess whether the extension is proportionate to the original house and plot. Very large extensions that significantly increase the footprint or mass of the original dwelling may be resisted even on a detached plot.

• •**Design and materials:** The extension should reflect the character and materials of the original house — or, where a contemporary approach is deliberately chosen, it should be clearly distinct and high-quality.

• •**Impact on neighbours:** Even though there are no shared walls, neighbouring properties can still be affected by overshadowing, overlooking, and visual impact. First-floor side windows adjacent to neighbouring properties require careful design.

**Costs for detached house extensions:**

• •**Single-storey rear extension (25–40m²):** £55,000–£100,000 construction cost
• •**Double-storey rear extension (25–40m² per floor):** £100,000–£180,000
• •**Side extension (single-storey, 20–30m²):** £45,000–£80,000
• •**Loft conversion (standard dormer, no party wall complications):** £50,000–£80,000
• •**Garden room or home office (detached, 25m²):** £30,000–£60,000

These are construction costs excluding professional fees, planning, and VAT.