Converting a House to Flats: Planning, Building Regulations, and What to Expect

**Why planning permission is required**:

Converting a house to two or more flats is a 'material change of use' — specifically, a change from Use Class C3 (dwellinghouse) to Use Class C3 (two or more self-contained dwellings). Despite both being within C3, the subdivision of one dwelling into multiple dwellings requires planning permission under the TCPA 1990 — it is not Permitted Development.

*Exception: where a house was previously converted to flats and reverted to single dwelling*: In some cases where a property was previously flats and has been used as a single dwelling for less than 10 years, returning it to flats may not require a new planning permission. This is a complex area — confirm with the LPA or a planning consultant before relying on this exception.

**What the LPA assesses for a flat conversion application**:

*Local housing need and density*: Most London boroughs welcome flat conversions in principle — they increase the housing supply and provide smaller, more affordable units. However, some boroughs have specific policies restricting conversions in areas of high flat concentration or where the loss of larger family homes is a concern. Always check the LPA's Local Plan for specific policies on 'subdivision of dwellings' or 'flat conversions'.

• *Minimum internal space standards (London Plan)*:
• The London Plan (2021) and most London LPAs require minimum Gross Internal Area (GIA) standards for new dwellings:
• •Studio flat: minimum 37m²
• •1-bedroom flat (1 person): 39m²
• •1-bedroom flat (2 persons): 50m²
• •2-bedroom flat (3 persons): 61m²
• •2-bedroom flat (4 persons): 70m²

Conversions that produce flats below these minimum sizes will be refused in most London boroughs. For a typical London Victorian terrace (floor area of approximately 80–100m² per floor), a two-flat conversion (ground floor + lower ground floor as one flat; upper floor(s) as the second flat) can usually achieve the minimum standards. Three or more flats from a standard terrace are often very difficult to achieve above the minimum size standards.

*Amenity space*: London Plan standards require a minimum external amenity space provision — typically 5m² per habitable room for flats without direct ground-level access, or a private garden for ground-floor flats. For rear-garden flats (the lower flat in a conversion), the garden can satisfy this requirement. For upper-floor flats, a balcony or roof terrace (added as part of the conversion) may be required.

*Separate access arrangements*: Each flat must have independent access — either a shared entrance hall with separate flat entrance doors, or a separate side/rear access for the ground-floor flat. The access arrangement must be shown on the planning drawings.

*Parking and cycle storage*: Some LPAs require a minimum number of parking spaces per flat (particularly in suburban boroughs); others (inner London) require car-free or limited-parking development. Cycle storage (minimum 1 space per flat, typically a secure bin store or covered lockable store) is commonly conditioned.

**Planning application requirements for flat conversions**:

• •Location plan (1:1250)
• •Block plan (1:500)
• •Existing and proposed floor plans showing the subdivision arrangement, room dimensions, and flat boundaries
• •Existing and proposed elevations (particularly if new windows, rooflights, or external doors are proposed)
• •Design and Access Statement addressing LPA policies, London Plan space standards compliance, amenity space provision
• •Transport Statement or Parking Survey (if required by the LPA)

*Planning application fee*: £206 per flat created (England, 2025) — a two-flat conversion from one house costs £206 (one new flat created); a three-flat conversion costs £412.

*Planning determination period*: 8 weeks (statutory); 10–14 weeks typical in London for a straightforward conversion.

**Overview of relevant Building Regulations Parts**:

A house-to-flat conversion must comply with the Building Regulations in full — not just for the new works but, where Building Control requires it, for the existing structure being converted. Key parts for flat conversions:

*Part B — Fire Safety*: Fire safety is the most critical technical aspect of a flat conversion. The requirements depend on the number of flats, the height of the building, and whether the flats share a common area (entrance hall, staircase):

• •Escape routes: each flat must have a safe means of escape in the event of fire — typically a protected escape route (30-minute fire-resisting construction) from each flat's entrance door to the final exit of the building
• •Fire doors: each flat entrance door = FD30S (30-minute fire door with smoke seal); doors onto the escape staircase = FD30S; doors between the escape stair and any risk rooms = FD30S
• •Compartmentation: the separating floor between the flats must be REI 60 (60-minute fire resistance) — for Victorian timber joist floors, this typically requires upgrading the floor construction (additional plasterboard ceiling layers, sometimes mineral wool between joists)
• •Fire detection: Grade D (mains-powered, battery backup, interlinked) smoke detectors in each flat (LD2 coverage — living rooms, hallways, bedrooms); heat detector in each kitchen; interconnection between all devices in the common parts
• •Basement flat: if the lower flat has its only exit through the ground floor (and the ground floor is the other flat's space), an alternative means of escape (door to garden, escape window) may be required

• *Part E — Sound Insulation*:
• Satisfactory resistance to airborne and impact sound transmission between flats is required. The performance standards (from Building Regulations Approved Document E) are:
• •Airborne sound insulation (separating floors): minimum 45 dB DnTw (normalised level difference)
• •Impact sound insulation (separating floors): maximum 62 dB LnTw (normalised impact sound level)

• Achieving these standards in a Victorian timber floor (which is the most common situation in London conversions) requires an upgraded floor construction. Typical approaches:
• •Platform floor (floating floor on resilient layer above existing): Regupol or similar isolation layer under chipboard deck; estimated improvement 15–25 dB
• •Resilient ceiling below (independent ceiling on spring-mounted hangers): estimated improvement 15–20 dB
• •Combined solution (both): typically achieves the required performance
• •Acoustic mineral wool between joists: 50–100mm mineral wool batts; improvement 5–8 dB (supplementary)

Note: A new Build and a conversion have different performance standards — a conversion only has to meet the 'Conversion standard' which allows testing to demonstrate compliance, rather than achieving a higher Robust Detail standard. Pre-completion acoustic testing (by an accredited tester) is usually required by Building Control for conversions.

*Part A — Structure*: The separating floor must be structurally capable of independently supporting the loads of the floor above without relying on the other flat's structure. Victorian timber joist floors (typically 4" × 2" at 12" centres for a 3.0–4.0m span, which is inadequate by modern standards) may require upgrading to support the loads required by Part A. A structural engineer must assess the existing floor construction and specify any upgrading required.

*Part J — Combustion Appliances*: If the conversion involves new or relocated boilers, each flat must have its own independent boiler and flue (or be connected to a communal heating system with metering). Gas boilers are not permitted in communal areas — each flat must have its own supply and boiler.

**Building Regulations application**:

A Full Plans application (drawings reviewed before work starts) is strongly recommended for flat conversions — the fire safety and acoustic compliance requirements are complex enough that design approval before work starts significantly reduces the risk of compliance problems on site.

**Typical conversion costs for a house to two flats (London 2025)**:

*Two-flat conversion of a Victorian mid-terrace (3 bedrooms, approximately 100m² per floor):*

• •Internal walls and partition work: £8,000–£18,000
• •Separating floor upgrade (acoustic and fire-resisting): £8,000–£18,000
• •Fire doors and fire stopping: £6,000–£14,000
• •Fire alarm and detection system: £2,500–£5,000
• •New kitchens (2 × kitchens; supply of units client): £6,000–£14,000 (installation and first-fix)
• •New bathrooms / shower rooms (2 × bathrooms; supply of sanitary ware client): £8,000–£16,000
• •Separate meters (electricity — each flat): £800–£2,000
• •Separate gas meters (each flat): £500–£1,500
• •Communal area lighting (timer/sensor controlled): £600–£1,500
• •Electrical upgrade (consumer units per flat, Part P): £3,000–£6,000
• •Plumbing upgrade (separate supply to each flat): £4,000–£8,000
• •External works (separate post boxes, door entry system): £500–£2,000
• •Planning application fee + Building Regulations fee: £700–£1,500
• •Professional fees (architect, structural engineer, acoustic consultant): £4,000–£10,000
• •**Total: £52,100–£117,500 (typical mid-range: £70,000–£90,000)**

**HMO licensing**:

A conversion that results in a house occupied by 5 or more unrelated persons in 3 or more storeys requires a Mandatory HMO Licence from the local authority. Many London boroughs also have Additional Licensing schemes that extend licensing requirements to smaller HMOs (3+ unrelated persons). Where the flat conversion creates self-contained flats (not rooms sharing facilities), the property is not an HMO — the licence requirement does not apply. However, if the lower flat is let to a family and the upper flat is let to 3 or more unrelated sharers, the upper flat may require a licence as a small HMO.

**Investment case — yield comparison**:

*Example: 3-bedroom Victorian terrace in London suburbs, market value £550,000*:

• As a single let:
• •Market rent: £2,200–£2,600/month
• •Gross yield: 4.8–5.7%

• As two flats (after conversion at £80,000 cost, uplifted value £640,000):
• •Ground floor 2-bed flat market rent: £1,400–£1,700/month
• •Upper floor 2-bed flat market rent: £1,400–£1,700/month
• •Combined monthly rent: £2,800–£3,400
• •Gross yield on uplifted value: 5.25–6.38%
• •Additional management and void cost: higher than single let

The investment case for a house-to-flat conversion is primarily capital growth (the two flats as individual saleable assets are worth more than the single house) rather than pure yield improvement. The separate flats can be sold individually, potentially to owner-occupiers at a premium over the investment value, unlocking significant capital gains relative to the conversion cost.