Loft Conversion Design Guide
Converting the loft is one of the most cost-effective ways to add extra living space to a home. New rooms within the roof can benefit from sunlight for much of the day, make the most of any views, and can have tremendous character due to the interesting roof shape.
At the top of the house the roof space can be the ideal location for a new master bedroom, a home office, playroom or even a self-contained annexe, but the new rooms can be suitable for almost any residential use you can think of, providing you comply with the Building Regulations.
Unlike most extensions, a loft conversion adds space without building over the garden. The only sacrifice is the space required to accommodate access via a new staircase.
A project typically takes four to six weeks to complete and it is usually possible to continue living in your home whilst the loft is being converted.
Assessing Potential
When assessing the potential of your loft for conversion, measure the space where there is a clear headroom of 2.0-2.1m or more between the joists and rafters. Once the floor has been built up and the roof insulated, this will leave you with around 1.9-2.0m of headroom, which is the minimum practical ceiling height.
Even if your loft does not currently offer enough space to make it viable for conversion – either because of the roof shape or height, the roof truss design or the position of cold water header tanks – there is probably a design solution that could make it possible. For example, increasing the volume of the roof can make a dramatic difference to the amount of usable space available.
Conversions are defined into different categories according to design and how any potential new space is added:
Rooflight Conversion
The existing roof space is converted with no increase to the volume other than the simple addition of rooflights to the front and back. Windows may also be added into the gable walls. This is the most cost-effective option as it involves minimal alteration. Costs range from £850-£1,250/m2.
Dormer Conversion
Dormer windows are added into the pitched roof plane to increase volume at the back or sides, and sometimes at the front. These may be relatively small extensions housing one or two windows (especially at the front facing the highway), but may be much larger across the whole of the roof width (usually at the back), forming a large area with full headroom. Costs range from £950-£1,650/m2.
Hip-to-Gable Conversion
When a roof slopes down to the eaves on all four sides it is known as a ‘hipped’ roof. To increase the usable space within a hipped roof, one (typically on a semi) or more hips can be replaced by a gable wall and the roof extended over the gables to create more volume. Costs range from £1,250-£2,250/m2.
Gable-to-Gable Conversion
Common in terraced houses, the gable walls are built up and the roof at the back rebuilt to increase the pitch so it is nearly vertical up to ceiling height, effectively forming a wall with windows, and then almost flat back to the ridge, forming a large area with full headroom. Costs from £1,250-£2,250/m2.
Mansard Conversion
This involves replacing the whole roof with a new box-like structure that effectively adds another full storey, with four almost vertical tile-hung walls, topped by a near flat roof. On a terraced house a mansard roof may span from gable to gable, front to back. Costs range from £1,250-£2,250/m2.
Replacing the Roof
Where there is currently a very shallow pitched roof with little or no usable space, it may make economic sense to remove the roof and replace it with a new structure that has a steeper pitch and more usable space. Costs range from £1,350-£2,350/m2.
Lowering Ceilings
In a Conservation Area alterations to the roof may not be permitted, especially increasing the ridge height. A solution is to add more space by lowering the ceiling in the storey below. Costs range from £1,550-£2,550/m2.
Meeting the Regs
Ceiling Height
There is no minimum ceiling height to meet the Building Regulations, other than above stairs, which require 2m of clear headroom. For a loft conversion this can be reduced to 1.9m at the centre of the flight and 1.8m at the edges to allow for sloping roofs.
The minimum practical ceiling height is 1.9m, but often attic rooms incorporate areas with sloping ceilings that are still usable for seating, bed heads, storage etc.
Insulation
If the existing roof covering (tiles, slates etc.) is being replaced it makes sense to insulate over the rafters and fit a breathable roofing membrane (waterproof, but air permeable to ventilate the roof structure). This creates what is known as a ‘warm roof’ and is very effective. Permitted Development rights usually allow for an increase in roof height of 150mm to accommodate insulation.
Where the existing roof covering is being retained, insulation will be required between the rafters (leaving a 50mm ventilation void) and beneath the rafters (usually with plasterboard bonded to it). Adding insulation under the rafters will reduce the ceiling height, so it makes sense to use the most space-efficient insulation materials possible, such as foil-backed rigid foam (i.e. Kingspan or Celotex). Many local authorities accept the use of thin multi-foil insulation materials, which are even more space efficient than insulation board.
Existing gable walls will also need to be insulated to meet the Building Regulations.
Soundproofing
It’s worth making an effort to reduce sound transfer between the new attic storey and the floor below — as the Building Regulations standard is not very rigorous. Sound can travel between the attic rooms and the floor below by two means: airborne transfer and impact transfer. The first can be reduced by making sure the structure is airtight, taping insulation materials, and using sealant around the floor edges and under skirting boards and floorboards. The voids between the floor joists can be filled with high-density acoustic insulation (acoustic mineral wool) — this cannot be used around recessed spotlights. Using high-density cement-impregnated chipboard will also help reduce impact transfer.
Staircases
The design and position of the staircase is a key decision in a loft conversion, as it will dictate how much of the storey below is lost to create access. The stairs must land where there is at least 1.9m of headroom (and 1.8m at the sides), which can limit options.
The steeper and narrower a staircase, the more space efficient it is. The maximum pitch under Building Regulations is 42° and although there is no minimum width for a staircase, less than 600mm is unlikely to prove practical. A spiral staircase is one of the most space-efficient options available.
Fire Regulations
Where your loft conversion forms a third storey, the structure needs to be separated from the rest of the house, and the walls, floor and doors given half-hour fire protection. This can be achieved using two layers of plasterboard and fire doors.
The means of escape in a fire is usually the main staircase, which must lead directly to an external door, so this too needs to be enclosed and given half-hour fire protection. This means that all doors to habitable rooms need to be fire doors. Hard-wired smoke alarms must also be added to each floor.
Where the staircase ends in an open plan room, the space can be fitted with a sprinkler system and a fire door separating the ground floor from the first floor level. Escape in the event of fire from the attic rooms can then be via a first floor window.
Loft conversions to bungalows do not need an enclosed staircase if there is a fire escape window in habitable loft rooms.
Planning Guide
Planning permission is not normally required for loft conversions, unless the roof space is extended.
Additions up to 50m3 (40m3 for terraces) to the side or rear of the roof, including dormers, are classed as Permitted Development (PD), so planning permission is not usually needed.
The insertion of rooflights usually falls within PD rights.
Planning permission is required for larger extensions, or roof alterations if PD is restricted (i.e. Conservation Areas).
Listed buildings will require listed building consent.
Any new side-facing windows must have obscured glazing and be non-opening, unless the parts of the window which can be opened are more than 1.7m above the floor of the room in which the window is installed.
The roof height can normally be increased by 100mm along the existing roof plane under PD rights to allow for the insertion of insulation, or rooflights.
The new roof must be no higher than the existing, and dormer windows must be set in from the verges by 200mm, except on gable-to-gable conversions.