Converting a House to Flats in London: Planning, Costs and Process

Yes — in almost every case, converting a house (Use Class C3) to flats (Use Class C3 or mixed) requires planning permission because it involves a material change of use. The conversion of a house into two or more self-contained dwellings requires a planning application.

• In addition to the change of use, the physical works typically include:
• •New entrance arrangements (separate front doors, communal entrance, or division of the existing entrance hallway)
• •New kitchen and bathroom in the upper flat (ground floor kitchen usually becomes the ground floor flat's kitchen)
• •Acoustic separation between floors
• •Fire separation between units
• •Separate utility meters and services

All of these changes are visible to Building Control and form part of the Building Regulations application.

• **Planning policy for flat conversions in London**:
• London boroughs have varying policies on house-to-flat conversions. Common concerns include:
• •Impact on family housing stock (many boroughs want to protect family homes and resist sub-division)
• •Overdevelopment of the site (if the conversion involves extensions or additional structures)
• •Parking impact
• •Impact on the character of the area

Some boroughs have adopted specific policies resisting flat conversions where they would result in the loss of family housing. Check the Local Plan for the specific borough before assuming permission will be granted automatically.

**Conversions of very large houses**: very large houses (typically 5+ bedrooms) are sometimes more readily approved for conversion — planners accept that the existing use as a single large family home is effectively impractical and conversion to flats represents a sensible use of the building.

**Freeholder consent**: if the property is leasehold, the freeholder's consent is required before any works — and the lease may prohibit subdivision. Always check the leasehold position before purchasing a property for conversion.

A house-to-flat conversion triggers a comprehensive Building Regulations application under the 'change of use' provisions. Key requirements:

**Part B — Fire Safety**: This is the most demanding element. The conversion creates two (or more) units with shared structure — the risk of fire spreading from one unit to another, or trapping occupants, must be designed out.

• Typically required:
• •60-minute fire separation between the two flats (floors and walls): FD30S fire doors opening onto communal areas, 60-minute fire-resistant floor construction between the units
• •Protected staircase: if there is a shared entrance and stair, it must be a fire-protected enclosure — FD30S fire doors, fire-resistant linings, automatic fire detection
• •Smoke alarms in each flat (Grade D1 interlinked LD2 minimum)
• •Emergency lighting in communal areas where daylight is insufficient in a fire
• •Addressable fire alarm in larger conversions

• **Part E — Acoustic Separation**:
• The floor between the two flats must meet airborne and impact sound insulation standards:
• •Airborne: Dnt,w + Ctr ≥ 45 dB
• •Impact: L'nT,w ≤ 62 dB

• In a Victorian or Edwardian terrace, the existing timber floor joists typically achieve nothing close to this. The acoustic upgrade required:
• •Resilient bars and 2 layers of acoustic plasterboard on the ceiling of the lower flat
• •Acoustic floor system (mineral wool between joists, acoustic mat, floating chipboard or engineered floor)
• •This adds approximately 100–150mm to the floor buildup — check head heights on both floors

**Part L — Thermal Performance**: New windows and external fabric elements must meet current U-value requirements.

**Part F — Ventilation**: Each flat must have adequate ventilation — kitchen and bathroom extractors, trickle vents or MVHR.

**Drainage**: separate or shared drainage must be agreed with Building Control. The ground floor flat's drainage is straightforward; the upper flat's kitchen and bathrooms drain via new pipework which must connect to the drainage system without affecting the lower flat.

**Gas and electrical**: separate consumer units and ideally separate gas meters for each flat simplify landlord/tenant relationships and energy billing.

The physical challenges of a house-to-flat conversion depend heavily on the layout of the existing building.

• **The key question: where does the division happen?**
• For a two-flat conversion of a typical 3-bed Victorian terrace, the most common arrangement is:
• •Ground floor flat: ground floor rooms (front reception, rear kitchen-diner, rear addition rooms)
• •Upper floor flat: all first floor rooms plus a new kitchen and bathroom (or using a rear addition room on first floor as a kitchen)

Alternative: vertical split (rarer in terraced houses) — dividing the building vertically with a new party wall.

**The upper flat kitchen problem**: The most common design challenge is where the upper flat's kitchen goes. In a standard 3-bed terrace, the first floor has only bedrooms and a bathroom — no obvious kitchen location.

• Solutions:
• •Use the small rear bedroom as the upper flat's kitchen (reduces bedroom count to 2)
• •Use part of the landing
• •Convert part of the rear addition roof space into a small kitchen
• •A loft conversion above the upper flat to create an additional bedroom and kitchen space (most expensive but adds most value)

**Shared entrance**: The ground floor flat should ideally have direct access from the front door. The upper flat typically accesses via the shared hallway and staircase. This shared entrance area must be fire-compartmented from both flats.

**Bin storage, post boxes, and cycle storage**: often required by the planning condition. Each flat needs a post box. Bin storage for two sets of bins requires planning — often at the side or rear of the property.

**Leasehold vs. freehold on sale**: when the converted flats are sold or let, they are typically sold on long leases with a shared freehold. The freeholder (usually the conversion owner) holds the freehold. Each leaseholder holds a 99–999 year lease. Consult a solicitor to structure the leasehold arrangement correctly before starting works.

Converting a 3-bed Victorian terrace to two self-contained flats is a substantial project. The works are more extensive than a standard refurbishment because of the fire separation, acoustic upgrade, and new services requirements.

**Typical all-in construction cost (3-bed terrace → 2 flats, London, 2026, excl. VAT)**:

• •Basic conversion (existing layout used, no extensions, functional finish): £65,000–£95,000
• •Mid-specification conversion (loft conversion for upper flat if needed, quality finish): £90,000–£140,000
• •Premium conversion with loft + kitchen extension: £130,000–£200,000+

• **Itemised cost breakdown (typical 3-bed → 2 flat conversion)**:
• •Structural opening / floor preparation: £5,000–£12,000
• •Acoustic floor upgrade (whole floor between units): £6,000–£14,000
• •Fire compartmentation (doors, linings, sealing): £8,000–£18,000
• •New kitchen in upper flat: £8,000–£20,000 (contractor supply and install; client supplies units)
• •New bathroom in upper flat: £6,000–£12,000
• •Full rewire (2 separate consumer units): £12,000–£20,000
• •New plumbing (separate services): £9,000–£16,000
• •Communal entrance works, new front doors, intercom: £4,000–£9,000
• •Replastering and making good: £6,000–£12,000
• •Decoration: £4,000–£8,000
• •External works (bin store, lighting, boundaries): £2,000–£6,000

• **Professional fees** (not included above):
• •Architect / planning consultant: £3,000–£7,000
• •Structural engineer: £800–£2,000
• •Planning application fee: £258 (change of use)
• •Building Control fees: £1,500–£3,500

**VAT note**: construction of a new dwelling converted from a non-residential building can qualify for zero-rate VAT. House-to-flat conversion (residential to residential) is typically standard-rated (20%) unless the building has been empty for 2+ years (reduced rate of 5% applies). Discuss with your accountant.

A house-to-flat conversion follows this sequence:

1. **Feasibility and design**: assess the existing building, establish the flat configuration, understand the head height on both floors after acoustic upgrade, identify kitchen location for upper flat. Typically done with an architect.

2. **Planning application**: submit Householder or Full Planning Application for change of use. Allow 8–12 weeks for determination. If the council requires a Heritage Statement or Design and Access Statement, these add to the cost and time.

3. **Building Regulations application**: Full Plans application submitted with drawings. Allow 4–6 weeks for initial check and conditional approval. Building Control must inspect at key stages.

4. **Party wall notices** (if applicable): if the upper flat's kitchen will share a wall with the neighbour, or if the acoustic works touch the party wall, notices may be required.

5. **Construction programme** (on a typical 3-bed terrace conversion): • Strip-out: 1 week • Structural works: 1–2 weeks • Acoustic floor system: 1 week • Fire compartmentation works: 1 week • First-fix plumbing and electrics (both flats): 3–4 weeks • New kitchen/bathroom upper flat: 2–3 weeks • Plastering: 2 weeks • Second-fix and decoration: 3–4 weeks • Communal area: 1 week • Total: approximately 14–20 weeks on site

6. **Building Control completion certificate**: essential. Without it, the conversion is not legally a separate dwelling for mortgage or sale purposes. Insist on obtaining this before practical completion.

7. **Separate utility connections**: arrange separate gas and electricity meters and connections with the utility providers.

8. **Legal work**: your solicitor creates the leasehold structure, registers the two separate titles at Land Registry, and registers the leases. This happens after practical completion.