Building Control Service
If the work involves any new accommodation in the roof space (bedroom, spare bedroom, study, occasional playroom, etc), or any structural alterations (adding or removing joists, installing a roof light if it involves trimming the rafters, etc), then you will need to make a Building Regulations application.
If it involves any alteration to the size or external appearance of the building, then Planning Permission may also be required.
What things do I need to consider?
The following guidance only applies to a loft conversion of no more than two rooms and a floor area of no more than 50m² in an existing one or two storey single-family dwelling house. For advice on any other type of building, please contact the Building Control section. The information in the following pages is NOT authoritative, and is for guidance ONLY.
A structural engineer should ideally check new structural members or alterations to existing structural members. This can often result in a more economic design than relying on “rule-of-thumb” sizing of members. The Building Control Surveyor checking the plan or the works on site may require calculations to show that the new structure is adequate, or that the work will not adversely affect the stability of the existing building.
Existing ceiling joists are likely to be inadequate to act as floor joists, therefore new deeper joists will be needed. Floor joists can be supported on load-bearing walls, or floor beams (see below).
Load bearing walls
Load bearing walls need to carry the loads directly to an adequate foundation, or to a beam or other structural member that will support the loads. They will generally be masonry walls, but timber stud walls may be used (subject to structural engineer’s calculations).
Any opening at lower levels in a load-bearing wall (e.g. a through lounge) requires an adequate lintel over. This may need to be exposed and checked by the Building Control Surveyor.
Beams to support floor joists can be made of solid timber, Glu-lam timber, steel, or a combination of timber and steel (a flitch beam). The exact size will depend on the loads on each beam and its span, but for spans of 4m or more, it is usually uneconomic to use solid timber sections.
The existing roof structure may be altered or upgraded as part of the conversion. Existing rafters, purlins or props may need to be removed to install the new floor or roof windows. The loads on the roof structure may increase (e.g. plasterboard ceiling to the underside of the rafters).
Where purlins or any intermediate props are removed, a new beam may be introduced under, or the size of the floor joists may be increased to carry the loads.
In a traditional roof design, the rafters are prevented from spreading by the ceiling joists or collars being securely tied to the rafters, and purlins acting as beams. If these are cut or removed (for example, when installing a dormer), then a ridge beam may be required, and if so, a structural engineer should be consulted.
Means of warning and escape
Mains-wired smoke detectors are needed in the hall, landing and stairs of all storeys.
There should be at least one detector on each storey, and the detectors interlinked (so that a detection of any one triggers all of them),
Each must be mounted within 7.5m of the door to any habitable room, and at least 300mm from walls and light fittings, and
They must be either wired to a separate circuit from the consumer unit, or have a battery back up.
The existing stairway (i.e., the hall at ground floor, the stairs from ground to the first floor and the first floor landing) at ground and first floor level should be enclosed within 30-minute fire resisting walls, including any glass panels. The stairway should not be open to any other room such as a lounge, kitchen, etc.
The stairway should open either:
- directly to a final exit, or
- to a space from which two escape routes are provided, each leading to final exits and separated from each other by fire resisting construction and fire doors.
All existing doors to habitable rooms at ground and first floors opening onto the hall or landing should be fitted with a self-closing device, and any new doors to the hall and landing to be fire resisting.
A new stair within an existing stairway enclosure should be separated from any new rooms by a self-closing fire door and fire resisting walls. Alternatively, the new stair may be separated from the existing storey by a fire door and fire resisting construction.
An emergency escape window is required to every new habitable room in the loft (in addition to the enclosure of the hall, stairs and landing described above).
This escape window, dormer or rooflight must have:
- a clear opening area of at least 0.33m²,
- a minimum clear opening width or height of 0.45m,
- the bottom of the opening no higher than 1.1m above the floor, and no more than 1.7m up the roof from the edge of the roof.
The window must be sited where a ladder can be placed below (e.g. not above an extension).
Surface spread of flame
Plastered walls and ceilings generally meet this requirement. Any timber finishes to the ceiling, and to the walls over a certain amount, will need to be treated with a fire-retardant paint or varnish (subject to Building Control approval).
The new floor and any structure supporting it must have a fire resistance of 30 minutes from underneath. This should extend to the inside face of the external walls.
The fire resistance of a floor is dependent on the ceiling beneath, the size and spacing of joists, and the boards over.
The existing lath and plaster ceiling below may provide the required fire resistance, provided that it is in good condition and firmly attached to the laths. If the ceiling is plasterboarded then 12.5mm thick boards are required to give 30 minutes fire resistance. You may have to provide an additional skim coat or additional plasterboard to the ceiling.
Joists to be at a maximum of 600mm centres, and at least 38mm wide (if the floor falls outside these limits you should consult Building Control),
Floor boarding over to be tongue and grooved, or plain edged boards overlaid with hardboard (to slow smoke and hot gases that penetrate the ceiling).
The first floor over any habitable rooms under the landing should also have 30 minutes fire resistance. If the existing floor is plain edged boards, or badly fitting tongue and grooved boards, this should be overlaid with hardboard.
The separating walls between dwellings should have at least 60 minutes fire resistance. Some older properties do not have separating walls within the roof space, or may have holes or gaps in them. These will need to be sealed up to the underside of the roof covering. You will also need to be careful of any new structural elements supported on this wall, or passing through it (if beams, floor joists or purlins pass through the wall, they can provide a route for smoke, fire and sound to spread from one house to another). Contact Building Control for advice.
External fire spread
Any dormer cheeks (the triangular external wall to the side) within 1.0m of a boundary to any other property will need to be either:
30 minutes fire resistant with not more than 2.0m² of combustible material added to the outside surface (e.g. timber or uPVC cladding), or
Not more than 1.0m².
There are requirements for thermoplastic rooflights, and for combustible materials used as roof coverings. For further advice on this refer to Building Control.
In a terraced or semi-detached house, the separating wall needs to be able to resist the passage of sound, irrespective of whether or not the neighbouring house has a habitable room in the roof (sound can travel through the wall into your neighbour’s roof void and through the ceiling, and vice-versa).
If the existing wall is
- 225mm (9″) coursed brick or stone,
- 2 leaves of 100mm thick brick or dense blockwork with 50mm cavity, or
- 2 leaves of 100mm thick aerated concrete blockwork with 75mm cavity,
then this will be adequate if lined with 12.5mm plaster or plasterboard. All gaps and holes in the wall need to be filled prior to plastering or plasterboarding.
If the existing separating wall is not adequate, then the best solution is a separate timber stud wall.
The existing wall should be rendered with sand & cement to seal any gaps,
50mm x 50mm timber stud wall, fixed only to the floor and roof structure (not to the wall) at least 13mm from the face of the wall.
The perimeter of the wall should be sealed with tape or mastic,
Mineral fibre at least 50mm thick between studs,
Lining of 2 layers of 12.5mm plasterboard on the room side only, with joints staggered.
If there is more than one room in the loft, then any of the following walls will need to be sound insulated:
- any wall between a bedroom and another room or bedroom; and
- any wall between a WC and another room (but not if the WC is an en-suite to a bedroom).
This can be achieved by installing mineral fibre (at least 25mm thick) between the studs.
The new floor needs at least the following sound insulation measures:
Floor boarding of at least 22mm thick chipboard, or at least 28mm thick softwood floorboards, and
100mm thick mineral fibre between the joists.
If there is a new ceiling under the floor, then it needs to be at least 12.5mm thick plasterboard.
As with thermal insulation, the mineral fibre should not be compressed to fit into a gap, as this reduces its effectiveness
Ventilation of rooms
A window with an opening equivalent to 1/20th of the floor area, and
Background ventilation (also called trickle- or night-vents) of 8000mm², which can be closed to prevent draughts.
Bathrooms or shower rooms (with or without a WC)
Mechanical extract fan of 15 litres per second capacity, and
- background ventilation of 4000mm², and an opening window (no minimum size), or
- the extract fan has a 15 minutes overrun, controlled by the light switch or a humidistat.
Sanitary accomodation (i.e., a WC)
- a window with an opening equivalent to 1/20th of the floor area, and background ventilation of 4000mm², or
- mechanical extract fan of 6 litres per second capacity with a 15 minutes’ overrun, controlled by the light switch or a humidistat.
If the new storey contains a kitchen or a utility room, or if there is an open-flued