Converting a house into flats or an HMO: the structural side

Do I need a structural engineer to convert a house into flats?

In almost every case, yes. Converting a house into flats is a material change of use under the Building Regulations, so Building Control will check fire safety, sound insulation between flats and the structure wherever it is altered. New stairs, removed walls, upgraded separating floors, loft rooms and new bathrooms all need structural calculations and drawings. An HMO conversion also brings Building Regulations and licensing requirements, though the amount of building work varies.

The starting point is the building itself. Many conversions, from Brent to Islington, are Victorian or Edwardian houses that have already been altered several times. The first job is to find out what is actually holding the building up before adding anything to it.

Planning and licensing: what applies

Converting a single house into self-contained flats needs planning permission. Councils look at flat sizes, the mix of units, amenity space, refuse and cycle storage, and the effect on the street.

For HMOs the position depends on size and location. A small HMO for three to six unrelated people can sometimes be formed without planning permission, but many London boroughs use Article 4 directions to remove that right in some or all areas. Larger HMOs normally need planning permission. Most HMOs also need a licence from the council, which sets its own standards for room sizes, facilities and fire safety.

Licensing and Building Regulations are separate regimes. A licence does not mean the building work has been approved, and Building Control approval does not mean the HMO meets licensing standards, so both need to be satisfied. Your council's planning and housing teams will confirm what applies at your address. The structural design should follow the approved layout, so settle the planning position first. A planning-stage structural feasibility note can flag structural issues before you commit to a scheme.

Floors: the heart of most conversions

In a converted house, the floors do more work than they used to. They become separating floors between flats, so they must resist fire and the passage of sound. That usually means adding layers and mass: extra plasterboard below, resilient ceiling systems, acoustic insulation between the joists, and sometimes a heavier floating floor above.

All of this adds load to timber joists sized more than a century ago for a single family, and often notched for pipes and cables since. A floor loading assessment checks what the existing joists can carry and whether they need strengthening, for example with new joists alongside, steel flitch plates or extra support.

New partitions are another common issue. A stud wall built on a floor with no wall beneath puts a line load on joists that were never designed for it. Kitchens and bathrooms add weight from fittings, tiles and screeds.

Stairs, openings and new layouts

Creating separate access to each flat often means a new or reconfigured staircase, a new stairwell opening through a floor, and sometimes a new entrance. Each of these cuts through joists or walls that carry load. Typical structural items include:

• Trimming joists around new stair openings
• Steel beams where load-bearing walls are removed or opened up
• Lintels for new or widened door and window openings
• Support for chimney breasts removed to gain space
• Loft conversions or roof extensions to form an extra flat
• Checks on foundations where new storeys or extensions add load

Fire separation and the structure

Fire safety drives much of the design. Flats need fire separation between them and a protected route out of the building, and the floors and walls forming that separation must hold their fire resistance for a stated period. The period depends on the height and layout of the building, and your fire strategy and Building Control will confirm it.

Structurally, this means that steel beams and columns supporting separating floors or escape routes need fire protection to the same standard, usually by boarding. Floors that carry other flats must not collapse before the occupants are out. The structural drawings show how each element achieves that, while the fire strategy itself is prepared by a fire specialist.

In terraces the party walls often carry the new beams, which means party wall notices. Listing those items early stops notice periods holding up the programme.

Existing defects to expect

Conversions also tend to expose problems the house already has. Sagging floors, joists cut for old pipework, rot at bearing ends in damp walls and floors that bounce noticeably are all common in older houses. These are best found and fixed while the ceilings are open, rather than covered up with new linings.

The survey regularly turns up issues that change the scope: a chimney breast removed downstairs with nothing holding up the one above, roof timbers spreading the walls, a rear addition moving away from the main house, or floors cut through by earlier plumbing. Dealing with these as part of the conversion is far easier and cheaper than coming back once tenants have moved in.

How we approach a conversion

Our conversion to flats or HMO structural package, from £1,200 + VAT, starts with a survey and appraisal of the existing building. We look at the walls, floors, roof and foundations, note past alterations and defects, and agree with you and your designer exactly what is changing.

We then design the new openings, stairs and floor strengthening, and provide structural input to the separating floors so the fire and acoustic build-ups can be carried safely. Calculations and drawings go through our three-step quality control process, with final sign-off by our Head of Engineering, before we send them to Building Control and deal with their queries.

Fire strategy, acoustic design and planning drawings are specialist work outside our structural package. Our drawings are set out so those specialists and your contractor can work from them alongside their own information.