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

A house and a flat are both in Use Class C3 (dwellinghouses) under the Town and Country Planning (Use Classes) Order 1987. This means a conversion from a house to flats is technically a change within the same use class — which might suggest no planning permission is required. However, the material change of use from a single dwelling to multiple self-contained dwellings has been interpreted by courts and planning authorities as constituting a material change of use requiring planning permission, and this is the approach taken in London. Additionally, many London boroughs have Article 4 Directions that specifically remove any permitted development rights for the subdivision of dwellinghouses. The London Plan and most London borough local plans include policies designed to protect the supply of family-sized housing (typically houses with three or more bedrooms) from conversion to smaller units. Applications to convert a family-sized house to flats in inner London are frequently refused on these grounds, particularly where the house has three or more bedrooms. Conversions of larger houses (four or five bedrooms or more) to two flats stand a better chance of approval, as the loss of family housing is considered less significant. Always check the local plan policies for the specific borough before committing to a conversion project, and consider obtaining pre-application advice from the planning authority.

An Article 4 Direction is an instruction from the local planning authority removing specific permitted development rights in a defined area. Many London boroughs have Article 4 Directions that restrict the creation of Houses in Multiple Occupation (HMOs — properties occupied by three or more unrelated people sharing facilities) without planning permission. The Use Class for small HMOs is C4 (houses occupied by 3-6 unrelated residents sharing facilities), and the change from a C3 single dwelling to a C4 HMO is a change of use. Article 4 Directions in boroughs including Lewisham, Newham, Waltham Forest, Haringey, and many others mean that this change of use requires planning permission rather than being permitted development. Larger HMOs (seven or more residents) are Sui Generis use and always require planning permission for change of use. It is important to distinguish between an HMO (shared house where residents share facilities but live together) and a conversion to self-contained flats (where each unit has its own kitchen and bathroom). Self-contained flats created by conversion always require planning permission in London regardless of Article 4 Directions, as noted above. Always verify the Article 4 Direction status and local plan policies for the specific borough and property before proceeding.

Even where planning permission is not required (rare for house-to-flats conversions), Building Regulations approval is always required for converting a house to flats. The key requirements are: Part B (Fire Safety) — the conversion must provide a protected means of escape for each flat, typically involving fire doors (FD30s) to all rooms opening onto the escape route, a fire-protected staircase, mains-wired interlinked smoke and heat alarms in each flat and the common areas, and if the building is more than two storeys, emergency lighting in the common areas. Part E (Resistance to Sound) — the floor and ceiling construction between flats must achieve a minimum sound insulation standard. For impact sound (footsteps, dropped objects), the maximum permitted weighted standardised level difference is 45dB; for airborne sound (voices, music), the minimum weighted sound reduction index is 45dB. Achieving these standards in Victorian and Edwardian construction typically requires significant upgrading of the floor construction — adding mass (concrete screed or additional layers), resilient layers (to decouple floor from ceiling), and acoustic ceiling systems. Part G (Sanitation and Hot Water) — each self-contained flat must have its own bathroom and kitchen, and the hot water supply to each flat should be arranged to prevent scalding and Legionella risk. Part J (Combustion Appliances) — if gas boilers are retained or installed, the flue arrangement for multiple flats requires careful design. Separate boilers and flues for each flat are generally required; sharing a boiler between self-contained flats requires specialist design.

Sound insulation between flats is one of the most technically demanding aspects of a house-to-flats conversion, and one of the most commonly underestimated costs. Victorian and Edwardian floor construction — typically 175mm timber joists with floorboards above and a lath-and-plaster ceiling below — achieves very poor sound insulation by modern standards, both for airborne sound (low mass, lots of gaps) and impact sound (rigid structure with no resilient layer). To achieve Part E standards, the floor construction between flats must be substantially upgraded. Common approaches include: acoustic mat plus screed — a resilient acoustic mat (such as Regupol or similar) is laid over the existing floorboards, followed by a 65mm floating screed, then the floor finish. The screed adds mass and the mat provides impact sound isolation. This approach raises the floor level by approximately 90mm, affecting door thresholds and potentially staircase dimensions. Independent ceiling system — an independent ceiling (completely decoupled from the floor joists above) is installed below the existing ceiling in the lower flat, using resilient hangers and acoustic mineral wool batts. This adds depth to the ceiling void (reducing ceiling height in the lower flat by 100-150mm) but provides excellent airborne sound attenuation without affecting the floor above. Combined approach — the most effective (and expensive) solutions combine a floating floor above and an independent ceiling below with acoustic mineral wool between the joists. Pre-completion sound testing (by an accredited acoustic engineer) is required under Building Regulations for all conversions.

Each self-contained flat must have separate gas, electricity, and water supplies to enable independent billing and to ensure safety in the event of a problem in one flat. Electricity — a separate consumer unit (fuse board) and meter for each flat is required. This typically means installing a new sub-main from the main incoming supply to each flat's consumer unit, and installing a sub-metering arrangement. In many cases a new incoming supply capacity is required from the network operator (typically UK Power Networks in London) if the existing supply is insufficient for multiple flats. Gas — separate gas meters for each flat where gas is used. The meter box may need to be enlarged (or a second meter box installed) to accommodate additional meters. In some conversions, removing gas entirely and converting to all-electric heating and hot water (heat pumps, electric boilers, or direct electric) simplifies the service separation and eliminates the complex flue arrangement issues that arise with multiple gas boilers. Water — a separate water supply stopcock and meter for each flat, taken from the incoming main in the basement or at ground level. The water supplier (Thames Water in most of London) must be notified and the meter arrangements agreed. These utility separation works typically cost £5,000–£15,000 for a two-flat conversion, depending on the extent of new infrastructure required.

Indicative total project costs (build + professional fees + planning + VAT) for converting a Victorian terraced house in London to two self-contained flats: Small terraced house (2 to 3 bedrooms, creating a ground floor 1-bed flat and a first/loft floor 1-bed flat): £60,000–£95,000. Medium semi-detached or mid-terrace (3 to 4 bedrooms, creating two 2-bed flats): £80,000–£130,000. Larger Victorian townhouse (4 to 5 bedrooms, creating two 2-bed or 3-bed flats): £100,000–£160,000+. These costs include: contractor build cost (all structural, acoustic, fire safety, mechanical and electrical, decoration, bathroom fitting, kitchen fitting); architect and structural engineer fees; planning application costs; Building Regulations fees; acoustic testing; and VAT at 20%. They do not include the cost of furnishing the flats, any ground rent or leasehold legal structure costs, or SDLT on any subsequent purchase. The financial return depends heavily on the value differential between the house value and the combined flat values in the specific location, and on the rental yield achievable if the flats are to be let. In many inner London locations, the combined value of two self-contained flats significantly exceeds the value of the original house, making conversion financially viable despite the costs.