An essential guide to loft conversions

Whether you’re dreaming of a master suite, a guest bedroom or a home cinema, a loft conversion is one of the easiest ways to extend – and add value to – your property

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As well as offering valuable extra living space, loft conversions give one of the best returns on investment you can get when it comes to home improvements. ‘Property experts frequently cite having a loft conversion as the project that will add most value to your home,’ says Becke Livesey, director at Econoloft. ‘It’s not unusual for our customers in Greater London to report added values of between £45,000 and £70,000.’

And because most loft conversions are allowed under permitted development rights, there’s no need to go through the lengthy process of obtaining planning permission. Here’s everything you need to know…

How much will a loft conversion cost?

Cost guide for a two-room, one-bathroom loft conversion*

Type of conversionCost
Mansard conversion£35,000–£45,000
Dormer conversion£30,000–£40,000
Hip to gable conversion£25,000–£30,000
Rooflight conversion£20,000–£30,000

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Planning Permission for loft conversions

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

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,’ says Becke. 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.


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.


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.

loft conversion with glazed dormer

As part of a whole-house renovation project, Erica and Andy Job decided to open up their loft to create their dream bathroom

Structural considerations

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.

Which type of loft conversion is best for my home?

There are four fundamental types of loft conversion and they vary in complexity and in cost. A typical loft conversion project will take four to six weeks to complete and most of the work can be done without disturbing the existing house. A more complex project that involves removing the existing roof will take eight to ten weeks.

Creating a new mansard roof

This is where the roof structure is altered at the back of the house (and sometimes at the front, too) to create a far larger area with full headroom. A mansard conversion typically spans from gable wall to gable wall and is like another full storey with almost vertical tiled walls and a flat roof. This results in an addition that may appear far more a part of the property and less like an add-on than a large box dormer. Costs range from £1,500-£2,500 per m².

The cross section of a mansard loft conversion

The cross section of a mansard loft conversion

Turning an attic into a room with rooflights

The existing loft space can be converted by simply adding rooflights, such as VELUX windows, plus upgrading the structure and adding stairs, electrics, plumbing, insulation etc. This is usually the simplest, quickest and cheapest type of conversion, as structural alterations are kept to a minimum. Costs range from £1,000-£1,750 per m².

The cross section of a roof light loft conversion

The cross section of a roof light loft conversion

The amount of useable space for a rooflight conversion will depend on the height and pitch of the roof. By the time the structure has been converted and insulated, only the area measuring 2.3m or more between the top of the floor joists and the underside of the rafters will have enough clear headroom for standing.

If your loft does not yield sufficient space for a simple rooflight conversion, you will need to consider one of the other design options shown on these pages to create the useable new room you need.

Dormer conversions

The roof structure is altered at the rear (or sometimes the sides) to add a large flat-roofed ‘box’ dormer, enlarging the amount of space with full-height headroom in the new room. The part of the roof being extended will need to be stripped and the structure rebuilt. Consequently, this option is more time-consuming and expensive. Costs range from £1,250- £2,000 per m².

The cross section of a dormer loft conversion

The cross section of a dormer loft conversion

Hip-to-gable loft conversion design

This usually applies to a semi-detached house or bungalow where the roof is currently hipped (sloped) to the side, as well as to the back and front. This roof is stripped and the hipped section removed. The end wall is then built up to form a new vertical gable and a standard pitched roof. The work creates a far greater area with full headroom. Costs range from £1,500-£2,500 per m².

The cross section of a hip-to-gable loft conversion

The cross section of a hip-to-gable loft conversion

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.

dormer loft conversion

Mick and Kym Catmull can now accommodate visiting grandchildren, thanks to a loft conversion that has added a storey to their home in Walton-on-Thames, Surrey

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.

*Figures supplied by

In the gallery: Plot the layout of the new loft room as soon as you know its dimensions – that way, you can ensure the furniture will sit (and fit) where you want it to, helping you to plan the wiring and lighting, too. Loft Concepts bespoke storage and furniture, from £3,000, Neville Johnson.

All prices and stockists correct at time of publishing.

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