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Loft Conversion Design Guide

Above: The extra space from a loft conversion can also present the ideal opportunity to create a ‘fun room’ such as this conservatory home bar.

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.

Choosing the Designer

There are two main options when it comes to designing your loft conversion. The first is to commission a designer, such as an architect or architectural technologist, to produce drawings which can be put out to builders on a competitive tender basis. You will be able to steer the designer to create exactly what you want, but fees for what is a relatively small project may be high as a proportion of overall costs, and you’ll also need to hire a structural engineer. The other option is to hand the whole project over to a design and build contractor, who will have a designer and engineer in their team and use standard design details and solutions. They might be less flexible and less creative in design terms, but a benefit is the all-inclusive price. Both options will deal with planning permission, if required, and Building Regulations approval.

Converting a Fink Truss Roof

Modern ‘fink’ truss roofs, made up of lots of smaller timbers held together with gang nail plates, are common on most homes built from the late 1960s and were previously considered unsuitable – or uneconomical – for conversion. However, there is a solution, which is to insert structural beams (usually steel) from gable to gable to carry the existing rafters, supported on further beams at floor level, with timber supports running between the two. This is a process that has become considerably more straightforward and cost-effective thanks to the development of a lightweight telescopic aluminium beam. Developed by TeleBeam (telebeam.co.uk), the system uses pre-approved ‘Type Approvals’, so can also remove the need for a structural engineer.


On one of the loveliest terraces in London’s Spittalfields, architect Chris Dyson has almost completely rebuilt his own Georgian townhouse, which was a stubby figure amongst its tall, elegant neighbours. As part of his extensive renovation project, Chris managed to raise the roofline to that of its neighbours’, undertaking a mansard conversion in order to accommodate a new small loft storey, which has been utilised as a sleek, contemporary kitchen in stark contrast to the remainder of the painstakingly restored interior. Chris Dyson Architects: 020 7247 1816

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.

Rooflights

The standard solution is to insert off-the-shelf rooflights that fit between the rafters, in line with the plain of the roof — usually slightly above so rainwater can be redirected around the opening. These are available in a wide range of sizes, including fire escape windows with an opening large enough to meet the Building Regulations.

A number of rooflights can be combined to form larger openings, adding design interest and maximising light and views.

Conservation Rooflights

In Conservation Areas, on listed buildings and period conversions such as barns, standard rooflights are often not considered acceptable and planners require the use of conservation rooflights.

Conservation-style rooflights are based on original metal rooflights from the Victorian era and are typically painted black with vertical glazing bars diving the glazing into two or more sections. The better quality versions are made from powder-coated steel, with thermally broken frames, and designed to sit flush with the plane of the roof.

Roof Lanterns

In place of rooflights, a section of roof can be fitted with fixed double-glazed units fitted over the rafters to form large fixed glazed areas relatively cost-effectively.

Another option is to add a raised roof lantern, which is like a small conservatory-style structure that replaces part of the ridge.

Michael's Key Tips

  • Add space by extending to make the conversion more viable and worthwhile
  • Install a space-efficient staircase. Spiral stairs, for example, will limit wasted landing space
  • Use eaves space behind the walls for storage, either cupboards or drawers
  • Fit a sprinkler system in order to carry out a loft conversion in an open plan house

Sort out the services: Lighting

As in any successful interior scheme, different light sources should be combined, including ambient (substituting for daylight), task (reading, working) and accent (to add atmosphere) lighting.

Lighting options on sloping ceilings include downlights and track lighting. A section of flat ceiling beneath the ridge or within a dormer window is the ideal surface for downlights. Where the ridge is higher, it may be possible to suspend pendants or a track lighting system.

Ambient lighting can also be provided using floor and table lamps, providing they are on a switched lighting circuit so that they can be controlled, and ideally dimmed from the main wall switches.

Ventilation

To maximise energy efficiency, the roof space should be made as airtight as possible, and to counter this it is essential to introduce controlled ventilation to prevent the risk of condensation and maintain good air quality. This means including background ventilation (airbricks and trickle vents) and rapid ventilation (via windows), plus extract ventilation in wet areas, such as bathrooms or a kitchen.

Attic bathrooms are not required to have a window providing the extract fan can provide rapid ventilation.

Heating

Extensions normally increase the heat load requirement of the house and so the boiler has to be upgraded, but a loft conversion may require little extra capacity as the space will be well insulated and can improve the overall energy efficiency of the house. However, if a bathroom is added, a boiler upgrade may be necessary. It is a good idea to switch to an unvented system that does not require header tanks but relies on mains pressure (as long as it’s at least 1.5bar).

Options for heat emitters in attic rooms include radiators, underfloor heating, or a combination of both, perhaps with electric underfloor heating mats in bathrooms.

Plumbing

If you are adding a bathroom you’ll need to think about the location of existing services. Adding hot and cold water supplies is straightforward, branched off the existing plumbing system either at the boiler or from the floor below. Flexible plastic plumbing is easy to thread through the joists.

Existing soil pipes are likely to be vented above roof level and it may be possible to boss a connection into this, or into another soil pipe on the floor below. Where there is no existing soil stack you may be able to add one; otherwise, a smallbore flexible waste pipe can be used to connect to the drains.

Bathroom Tips

If you are going to put a bedroom in the attic then it makes sense to try and fit in a bathroom, but do follow these tips:

    • Place a shower where there is full headroom
    • A bath can be tucked under the eaves
    • A WC ideally needs full headroom, as does a washbasin
    • A wetroom can be a space-efficient option, but needs full tanking
    • Use the voids in stud walls for concealed shower and tap mixers
    • Concealed cisterns in metal frames for building into studwork are ideal
    • Good lighting and large wall-to-wall mirrors create the illusion of space
    • Wall-mounted sanitaryware helps make a small bathroom appear more spacious

    2 Comments

    Fair play the lighting picture was amazing. Just be aware of how different the prices and variry betweeen builders. And also if it sounds to cheap to be true then it probably is!
    I live in a hundred year old semi detached house that does not have a loft space firewall between the two houses. I have recently been told that regulations have changed and that I would not be able to sell my property without one installed and that it is highly likely that my house insurance would not be valid. However, I have searched the internet and cannot find any information on this subject at all so I was wondering if you could help! Many Thanks,