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Attention to Detail


What will a loft conversion cost?

The cost will vary depending on size, but is usually between £30,000 and £50,000. A typical conversion with a rear dormer in a mid-terrace property costs around £35,000. You’ll find small companies will usually charge 10-15 per cent less than large companies.

Building Control Fees (around £500, plus VAT) are also payable by the homeowner to the local authority or a government-approved, independent inspection company, to check that the work is as contracted and to issue building regulation certificates to prove that it has been carried out in accordance.

Planning permission for loft conversions

In most cases, converting an existing loft space will not need planning permission, and loft extensions are allowed under permitted development.

Permitted development allows for a roof space increase of 50 cubic meters in detached or semi-detached homes, and 40 cubic metres in a terraced property. Restrictions apply when your home is listed or sits within a conservation area.

A common misconception is that you can build up to the permitted development allowance, then apply for permission for anything over that; but this won’t work. In many areas, the guidance for lofts converted under full planning permission is actually more restrictive than the allowable limit under permitted development, so weigh up what is possible before committing to a design.

Loft conversions are classed as permitted development and do not require planning permission, providing they meet the following conditions:

  1. Any new roofing must not exceed an additional 40 cubic metres of space on terraced houses.

  2. Any new roofing must not exceed an additional 50 cubic metres of space on detached and semi-detached houses.

  3. No extension must be made beyond the plane of the existing roof slope.

  4. No extension can be higher than the highest part of the roof.

  5. New roofing materials need to be like-for-like or close to original fittings.

  6. There must be no raised platforms or balconies.

  7. Side-facing windows must be set with obscured glazing and an opening 1.7-metres above the floor.

  8. For listed buildings or those in conservation areas, visit

Building regulations

Always remember that planning permission does not equal building regulations approval – the two have to be cleared separately. Every new conversion, including those done under permitted development, must comply with fire and building regulations. This covers the safety and quality of the building work, including:

  • checking the proposed structure is calculated properly

  • the safety of the stairs

  • insulation levels

  • effective drainage

  • electrical safety

Ask your local authority’s building control department, or a private sector approved inspector, to help early in the planning stages. A common pitfall is the need for a ‘direct means

Designing a loft conversion

Key considerations

  • Is it the best use of the space?

  • Is there space for furniture and storage?

  • Is the ceiling height acceptable?

  • Could the addition of a dormer create more space?

  • Does the position of the stairs optimise potential space?

  • Could more light be created with roof lights or light tunnels?


What about fire safety?

In an unconverted loft, the plasterboard ceiling in the upstairs rooms will delay the spread of fire to the roof space. However, when an opening is introduced for the staircase, safeguards must be in place to reduce the risk.

All habitable rooms in a loft conversion above a single-storey house should have an escape window measuring a minimum of 45cm high by 45cm wide, with an opening of at least 0.33m², and be no more than 1.1m above floor level.

For loft conversions above existing two-storey houses, more stringent provisions apply. There must be a 30-minute fire-resistant floor fitted, a 30-minute fire-resistant stair enclosure leading to its own final exit, and fire doors to all rooms expect bathrooms and WCs. Finally, at least one mains-operated smoke alarm with battery back-up must be installed in the circulation space of each storey.

Is my loft suitable for conversion?

Most properties will be suitable for a loft conversion so long as they have a loft that measures 2.3 metres at the highest point. As well as head height, other features that will help you decide whether your loft space is suitable for conversion are the pitch of the roof, the type of structure, and any obstacles, such as water tanks or chimney stacks.

Head Height

If the initial roof space inspection reveals a maximum head height of less than 2.3 metres, there are two solutions available, both of which will require professional input: You could remove all or part of the roof and rebuild it to the required height and structure; however, this is costly and requires getting planning permission. You’ll also need to protect your house from the weather during the works using a covered scaffold structure.

Alternatively, you could create height by lowering the ceiling of the room below, providing you maintain a height of at least 2.4m. Removing the existing ceilings is a messy job and a plate will need to be bolted to the wall for the new floor joists to hang from. There will also need to be a tie between the new ceiling and roof to prevent the roof spreading.

Take a measurement from the bottom of the ridge timber to the top of the ceiling joist; the useable part of the roof should be greater than 2.2m.

If you have appointed an architect or designer, invite them to illustrate clearly how much headroom there will be across the floor in the finished space. Some people are disappointed by how much standing space they actually have, and this isn’t always conveyed on plans.

The Building Regulations impose no minimum ceiling height for habitable rooms. The headroom standard for stairs of 2m applies, but this can be relaxed to 1.9m or 1.8m on the edge of a stair if necessary.


The higher the pitch angle, the higher the central head height is likely to be. If dormers are used or the roof is redesigned, the floor area can be widened.


There are two main types of roof construction — traditional framed and truss section. The traditional framed type is typically found in pre-1960s houses where the rafters, ceiling joists, and supporting timbers are cut to size and assembled on site. This type of structure is usually the most suitable for conversion as it can be easily, and relatively inexpensively, opened up by strengthening the rafters and adding supports.

Post-1960s, the most popular form of roof construction is factory-made truss sections, which mean the entire roof can be erected and felted in a day. Thinner – and therefore cheaper – trusses are used that usually have no loadbearing structures beneath them. Opening up lofts with this kind of structure requires added structural input, most commonly from the addition of steel beams. This requires skill, knowledge and equipment, and is therefore costly.

Will I need new joists?

Your existing ceiling joists are unlikely to be able to support the loft conversion floor; so extra joists will need to be added to comply with building regulations. A structural engineer will look at the separation distance needed between joists to support the anticipated load weight, and then specify the size and grade needed.

The new joists will run alongside the existing joists and span between load-bearing walls. They will normally be raised slightly to prevent them from touching the ceiling plaster below.

Above window and door openings, thicker timbers will be used to bridge the gap, so that pressure is not put on the existing lintel. Rolled Steel Joists (RSJs) may also be needed to distribute the load.

Insulation for loft conversions

There are two main ways of insulating the roof structure, and your Building Control inspector will specify which type you require.

Cold roof

The first method, called ‘cold roof’ insulation, can be carried out by a DIYer. It involves filling the space between the rafters with 7cm-thick slab foam insulation, ensuring that there is 5cm space between the roof felt and the insulation to allow for ventilation. A 3cm-depth of slab insulation is then attached to the inside of the rafters, giving a total of 10cm of insulation. The roof section of the loft conversion will require 30cm of mineral wool insulation, or 15cm of slab foam insulation, such as Celotex.

Warm roof

The other main method is ‘warm roof’ insulation. This involves fitting 10cm of slab foam insulation over the top of the rafters and adding a capping, followed by the tile battens and tiles. This is only a practical solution when the roof covering has been stripped off, such as where a dormer is being created. The dormer walls can be insulated with 10cm-thick slab foam insulation between the studwork. Plasterboard is attached to one side of any internal partition walls, a 10cm-thick quilt of insulation added, and then plasterboard added to the other side. Insulation is also placed between floor joists, and this is typically 10cm-thick.


How do I choose a staircase?

The best place for your staircase to land is in line with the roof ridge, which will make best use of the height available. The minimum height requirement above a staircase pitch line is two meters. In reality, the actual location of your staircase will depend on the layout of the floor below, and where the necessary height can be achieved using a dormer, rooflight or, if appropriate, converting a hip roof end to a gable.

Number of steps

Building regulations specify that the maximum number of steps you can have in a straight line is 16 – the average loft conversion normally only requires 13 steps.

Size of steps

The maximum step rise is 22cm and the minimum depth is 22mm. Any winders, steps that go around a corner, must have a minimum of 5cm depth at the narrowest point. The width of steps is currently unregulated.

Rules for balustrades

The minimum height of any balustrade is 90cm above the pitch line. Any spindles must have a separation distance that a 10cm sphere cannot pass through.

Picking windows and dormers for loft conversions


Rooflights are the most straightforward way of bringing natural light and ventilation to your loft conversion. The surrounding area is reinforced before the rafters are cut to make way for the rooflights. The rooflight frame is fitted within the opening, and flashings are added before making good the surrounding tiling. This type of window is the most cost-effective, and most likely to be allowed without planning permission, under permitted development rights. Conservation rooflights, which are slightly more flush to the roofline and made of metal, can also be installed.


Dormers not only give natural light, but can add space to a loft conversion, too. They are particularly effective where the pitch angle is high, as they can help increase the useable floor space. Dormers are normally installed by opening up the roof and cutting the required timbers to size on site. However, some loft conversion companies will make the dormers off site in their workshop and lift into place, which allows quick installation, and weatherproofing.

There are various types of dormer, from the standard ‘box’ which projects out with a flat roof, to the hip-to-gable, which is used on end-terrace or semi-detached houses to replace a previously sloping roof (a hip) with a wall that is flush to the exterior wall (forming a gable). The mansard type, most commonly seen on London terrace houses, also maximizes available roof space because it projects the maximum available head height, giving a greater usable floor area.

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