Double-Storey Extension — Planning, Costs & What's Involved

A double-storey extension is, in structural and planning terms, two storeys of new building added to the rear (or side) of an existing house. The typical project includes:

**Ground floor**: an extended kitchen-dining-living room, a utility room, or a playroom. The same design considerations apply as for a single-storey rear extension.

**First floor**: one, two, or occasionally three new bedrooms with en-suite bathrooms. In most London terraced houses, a double-storey rear extension adds one large bedroom with en-suite — in some cases two smaller bedrooms.

**Structural works**: foundations (often strip or piled), ground-floor slab, structural walls, first-floor structure (typically timber or steel frame), roof (matching the existing or with a different design), and the structural connections to the existing house.

**Internal works**: new staircase from the existing first-floor landing to the new first-floor bedroom(s); connection from the existing house into the new extension at both ground and first-floor levels (structural openings); full M&E (electrics, heating, plumbing) throughout.

**External envelope**: brick or render external walls; windows and doors; roof treatment.

• A double-storey extension always requires planning permission. It is not Permitted Development. This is because a two-storey extension:
• •Exceeds the single-storey Permitted Development height limit
• •Typically has a rear wall closer to the rear boundary than Permitted Development allows for two-storey works
• •Often affects the privacy of neighbouring properties at first-floor level (overlooking is a key planning concern)

**Planning application process for a double-storey extension**: 1. Appoint an architect to prepare planning drawings 2. Submit a Householder Planning Application to the local authority (fee: £258 for England as of April 2024) 3. 8-week statutory determination period (often 10–13 weeks in practice) 4. If approved: proceed to Building Regulations technical stage 5. If refused: appeal or redesign options

• **What planning officers assess**:
• •Impact on neighbouring properties — overshadowing, loss of light (45° rule), overlooking and loss of privacy
• •Design and materials — visual compatibility with the existing house and street
• •Impact on the 45° daylight envelope of neighbours
• •Rear boundary setback — most councils require at least 10.5m from the new rear wall to the opposing rear boundary (the 45° rule at first floor)

• **Common reasons for refusal or design modification**:
• •Insufficient rear boundary distance for the height proposed
• •First-floor rear windows creating overlooking into neighbouring gardens or rooms
• •Design that is visually dominant or incompatible with the host property
• •Works affecting a boundary wall or party wall that requires careful design

Double-storey extension costs in London (2025):

**Rear double-storey extension — standard specification**: 3m wide x 4m deep (2 storeys): £85,000–£115,000 4m wide x 4m deep (2 storeys): £105,000–£140,000 5m wide x 4m deep (2 storeys): £120,000–£165,000

**Rear double-storey extension — premium specification** (full-height glazing, rooflights, premium kitchen, high-spec bathrooms): £160,000–£220,000+

• **Key cost variables**:
• •**Width and depth**: the primary driver of cost. Each additional metre of width adds approximately £18,000–£25,000 across two storeys
• •**Roof design**: a simple pitched or flat roof costs less than a complex parapet or design-led roof. A matching pitched roof to the host property is typically the planning authority's preference and adds some cost vs a flat roof
• •**Ground conditions**: if ground investigation reveals poor bearing capacity, a piled foundation adds £10,000–£30,000 to the cost
• •**Party wall complexity**: abutting neighbours on both sides (terraced houses) increases party wall costs and programme risk
• •**Internal specification**: a premium kitchen and en-suite specification adds £20,000–£60,000 to the base build cost
• •**Structural opening size**: large structural openings between the extension and the existing house require bigger steels and more structural engineer input

These figures are for the build only. They exclude planning and architectural fees (£5,000–£15,000), structural engineer fees (£2,000–£5,000), and Building Control fees (£1,500–£3,000).

• **Party wall**: a double-storey extension on a terrace almost always triggers the Party Wall etc. Act 1996. The key notices are:
• •Section 2 Notice: for works to or at the party wall (within 3m of the party wall at two storeys requires Section 6 notice for excavations as well)
• •Section 6 Notice: if new foundations are within 3m of the neighbouring building and deeper than their foundations

At least 2 months must be given before starting works where a Section 2 notice is served. If the neighbour disputes the works, a Party Wall Award is required — adding 1–3 months and costs of £1,500–£3,000 per side.

**Overlooking mitigation**: planning conditions for first-floor rear windows are common. Where full-height windows are proposed that overlook neighbouring gardens, planning authorities often require obscure glazing, limited opening lights, or high cill heights. This must be designed in at planning stage.

• **Programme**:
• •Pre-construction phase: 16–24 weeks (planning 8–13 weeks + post-permission technical design + Building Regs + party wall)
• •On-site programme: 18–26 weeks
• •Total project duration from first meeting to completion: 10–16 months for most double-storey extensions in London

A double-storey extension is typically the highest-cost-per-project home improvement for a London terraced house, but also one of the most significant in terms of value uplift:

• **Why double-storey adds exceptional value**:
• •It adds both floor area (which increases the price per square metre basis of the property) and bedroom count (which moves the property into a higher price bracket)
• •Adding a 4-bed to 5-bed conversion, or a 3-bed to 4-bed with an en-suite, can move the property into a significantly higher buyer demographic
• •In London's market, every additional bedroom typically adds £30,000–£80,000 to value (varies significantly by location)

• **Typical value uplift in London (2025)**:
• •Double-storey rear extension adding 1 bedroom + open-plan ground floor: £80,000–£150,000 uplift
• •Double-storey rear extension adding 2 bedrooms + open-plan ground floor: £120,000–£200,000 uplift

**The investment case**: in many London boroughs, the construction cost of a double-storey extension (£100,000–£165,000) is recovered in full by the value uplift — and in high-value inner London locations, the return on investment is positive. This makes double-storey extensions attractive not just for lifestyle but as investment-led improvements, particularly for landlords upgrading rental portfolios or homeowners planning to sell.