Skip to main content

Loft Conversion: Beginner's Guide to Extending Up

Dormer loft conversion
(Image credit: Simon Maxwell)

A loft conversion is among the most popular home improvement projects in the UK for easing space pressure on the rest of the home. It’s a great solution for properties in urban areas or where a two-storey extension doesn’t make financial sense.

Loft conversion costs start at around £15,000 for a single ‘room-in-the-roof’ conversion, up to around £55,000 for a ready-made room that is manufactured off-site and then craned into position. 

Nationwide has estimated that an additional bedroom and bathroom could add around 20% to the value of a three-bed, one bathroom house (but do be aware of area price ceilings). This could equate to £46,000 based on an average house price of £230,292 (average house price in June 2019, Office of National Statistics).

Most loft conversions tend to fall under Permitted Development, but you will need planning permission if you live in a listed building or a designated area. You will also need to adhere to the Building Regulations.

You will also have to consider your existing roof structure and the need for a new staircase, as well as how you will light, heat and ventilate the new room(s).

This guide will explain the process and highlight the different options available and their costs.

If you are looking for ideas for your new space, check out our gallery of great loft conversion ideas.

How Much Does a Loft Conversion Cost?

Your loft conversion cost will depend on multiple factors including the existing roof structure and pitch, specification and the extent of remodelling work or alterations needed to accommodate the new staircase.

Here, we’ve listed the different types of loft conversion and their approximate costs:

Room in the Roof Loft Conversion

Prices start at around £15,000 for a room in the roof loft conversion, which is the cheapest and most straight forward option.

This will usually involve:

  • the reinforcement of the floor
  • a couple of skylights
  • added insulation
  • a staircase to the loft
  • electrics, lighting and heating
  • fire safety measures to comply with Building Regulations such as fire doors and smoke alarms.

Dormer Loft Conversion

This is as the above, but with the addition of dormer windows. This will increase the usable floorspace and can be used to add head height which gives you more options when it comes to placement of the stairs.

A dormer loft extension will cost upwards of £20,000. However the average dormer loft conversion with a double bedroom and en suite costs about £35,000–£45,000.

Changing the Roof Structure for a Loft Conversion

This option is the most expensive as it requires the complex removal and rebuild of the existing roof. This could be a hip-to-gable loft conversion or a mansard conversion, for example. You will need to hire a designer and secure planning permission.

This type of work is likely to cost upwards of £40,000.

Ready-made room options will cost around £55,000 for the average home. These are fabricated off-site and then craned into position.

Do I Need Planning Permission for a Loft Conversion?

Always check with your local planning office before you start any work.

Not always. In most cases, loft conversions tend to be considered Permitted Development (PD), but your design will need to adhere to a number of specified parameters.

If you plan on extending beyond the limits and conditions of PD, or your property is listed or located in a conversation area, then you will need to apply for planning permission. You will also need planning permission if you are altering the roof height or shape (which may be the case if you have to raise it for headroom).

Rooflights and dormers can be installed under PD, but they must not sit forward of the roof plane on the principal elevation, nor must they be higher than the highest part of the existing roof.

Loft Conversion Building Regulations

When converting a loft, you will need Building Regulations approval. A building control surveyor will inspect your conversion at various stages and will be responsible for issuing a completion certificate upon final inspection.

If your home is semi-detached or terraced, then you will need to notify your neighbours of your planned work if it falls under the requirements of the Party Wall Act.

Check out our latest subscription offer!

For even more advice, information and inspiration delivered straight to your door, subscribe to Homebuilding & Renovating magazine.

When it comes to a loft conversion, you are most likely to be concerned with Parts L, K, B and P of the Building Regulations.

Part L of the Building Regulations requires U-value targets for thermal efficiency to be met when you convert your loft into habitable space.

Part K concerns preventative measures from falling, collisions and impact, and requires a minimum headroom of 2m for all escape routes, including the stairs (although the rules are relaxed a little for staircases providing access to a loft conversion).

Parts B and P are concerned with fire and electrical safety respectively. Complying with Building Regs’ requirements on fire safety can be complex. In two or more storey homes, where an escape window would be more than 4.5m from the ground level, a ‘protected’ staircase needs to lead down to an exterior door — which can cause some issues if your staircase rises from a room, rather than the hallway on the ground floor, or your ground floor is open plan. There are typically solutions in both instances, but this area needs consideration with your design and/or build team.

Can All Lofts be Converted?

Assessing your loft space’s suitability for conversion involves considering numerous factors, including:

  • available head height
  • roof pitch
  • roof structure
  • obstacles such as water tanks or chimney stacks
Top Tip

Ask your designer to clearly illustrate 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 easily conveyed on plans.

Measuring Head Height for a Loft Conversion

When you measure from the bottom of the ridge timber to the top of the ceiling joist, you need to have at least 2.2m of usable space for a conversion to be suitable.

While the Building Regs impose no minimum ceiling height for habitable rooms, you will need to factor in the 2m headroom required for stairs (although you could relax this to 1.8m on the edge of the stair if needs be).

If the initial roof space inspection reveals a head height of less than 2.2m, there are two available – but costly – solutions that will require professional input.

Kitchen located in mansard loft conversion

As part of his renovation project, architect Chris Dyson completed a mansard conversion in order to accommodate a new small loft storey, which has been utilised as a sleek, contemporary kitchen (Image credit: Nigel Rigden)

Solution 1: Raise the Roof

This is structurally feasible, but the major problems are the high cost and getting planning permission approval. If the whole roof area needs removing, a covered scaffold structure, to protect the house from the weather during the works, would also be required.

Solution 2: Lower the Ceiling in the Room Below

This will require all the existing ceilings in question to be removed, causing much mess. With this method a plate will need to be bolted to the wall using shield anchors or rawlbolts, for the new floor joists to hang from. There is also a need for a suitable tie between the roof structure and the dwarf wall formed, to prevent the roof spreading.

You will also have to assess whether the space you are gaining in the loft makes up for the space you are losing in the rooms below.

(MORE: How to convert your loft if you have a difficult roof construction)

The higher the angle of the roof pitch, the higher the central head height is likely to be, and if dormers are used or the roof is redesigned, the floor area, and potential for comfortable headroom, can be increased.

Traditional frame type roof structures are often the most suitable type for loft conversions, allowing the space to be opened up relatively easily and inexpensively. The rafters may need to be strengthened or additional supports added (your structural engineer will advise on what is required).

Trussed roofs require greater structural input, normally involving the insertion of steel beams between loadbearing walls for the new floor joists to hang on and the rafter section to be supported on — together with a steel beam at the ridge.

Without the roof space for water tanks and plumbing, the heating and hot water system may have to be replaced with a sealed system.

Unvented hot water cylinders make a better choice than replacing the boiler with a combi boiler, but they do take up a cupboard-sized room, which you will have to find space and budget for.

Do I Need a Designer for my Loft Conversion?

While you can design a loft conversion yourself, employing the services of an architect or designer is advisable. Another alternative is a design and build company.

You have two main options (both will deal with planning permission, if required, and Building Regulations approval):

Taking this route means that you can steer the design to your exact, bespoke specifications, but remember that, as this is likely to be a relatively small project, the design fees are likely to be a high percentage of the overall costs.

Your architect or designer will produce drawings which you then put out to tender, and you may find that you need to also hire a structural engineer.

(MOREHow to find the right architect)

For an all-inclusive service (and price) then a design and build contractor is a great option. Design solutions and details are more likely to be standardised so you may find that you have less creative flexibility. 

Loft converted into master suite in this Victorian semi

This loft was converted as part of a whole house renovation project and features a new master suite with French doors leading out onto a roof terrace (Image credit: Jeremy Phillips)

Editor’s note: If you’re after information to help choose a loft conversion vendor that’s right for you, fill in the questionnaire below and we can provide you with information from a variety of vendors for free:

Adding a Staircase to a Loft Conversion

The ideal location for a staircase to land is in line with the roof ridge: this will make best use of the available height above the staircase.

The minimum height requirement above the pitch line is 2m, although this could be reduced to 1.9m in the centre, and 1.8m to the side of a stair.

In practice, the actual position will depend upon the layout of the floor below, and where necessary the available height can be achieved using a dormer or adding a rooflight above the staircase or, if appropriate, converting a hip roof end to a gable.

  • Maximum number of steps: the maximum number of steps in a straight line is 16 (typical installation usually only requires 13 steps)
  • Step size: the maximum step rise is 220mm, whereas the step depth or ‘going’ is a minimum of 220mm; these measurements are taken from the pitch point. The step normally has a nose that projects 16-20mm in front of the pitch line. However, the ratio of size must not exceed the maximum angle of pitch requirement of 42°. Any winders must have a minimum of 50mm at the narrowest point. The width of steps is unregulated, but in practice the winders are likely to limit the reduction in width.
  • Balustrading: The height minimum is 900mm above the pitch line, and any spindles must have a separation distance that a 100mm sphere cannot pass through.

(MOREStaircase Design Guide)

Do I Need to Replace the Ceiling Joists When Converting a Loft?

In most cases, additional new joists will be required to comply with the Building Regulations as existing ceiling joists are unlikely to be able to take a conversion floor.

Your structural engineer will specify the size and grade required.

The new joists span between load-bearing walls, and are normally raised slightly above the existing ceiling plasterwork by using spacers below the joist ends. This spacing must be sufficient to prevent any new floor joist deflection from touching the ceiling plaster below.

The new joists run alongside the existing joists. Above window and door openings, thicker timbers are used to bridge the opening, so that pressure is not put on the existing opening lintel.

Rolled steel joists (known as RSJs) are also specified to distribute the load, and in some installations are used to carry the ends of the new joists. If head height is limited, then thicker joists, more closely spaced, can be specified.

Bringing Natural Light into a Loft Conversion

Home bar in loft conversion

These homeowners have created a bar area in their loft space (Image credit: Jeremy Phillips)

You have two feasible options when it comes to bringing in natural light — rooflights or dormers.

The most straightforward method is to use rooflights that follow the pitch line of the roof. This type is fitted by removing the tiles and battens where the rooflight will be fitted. The rafters are cut to make way for the rooflight after suitably reinforcing the remaining rafters.

The rooflight frame is then fitted and flashings added before making good the surrounding tiling.

This type of window is the most economic, and more likely to be allowed without planning permission.

Dormers not only give natural light but can add space to a loft conversion. They are particularly effective where the pitch angle is high, as the useful floor area can be increased.

The mansard type will give maximum conversion roof space because it projects the maximum available head height, thus giving a greater usable floor area. A hip to gable conversion has a similar effect.

Dormers and other similar conversions are normally installed by opening up the roof, and cutting the required specified timbers to size on site.

Some loft conversion companies will make the dormers off site in their workshop and lift into place. This process allows quick installation, and quick weatherproofing.

Adding Artificial Lighting in a Loft Conversion

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.

Heating a Loft Conversion

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.

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

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).

(MORE: Guide to Underfloor Heating)

New loft space created by extending

Jo Dyson created a new loft space by extending, creating a light-filled space that houses an open-plan living room, kitchen and dining area. Jo added multiple rooflights, sash windows and bi fold doors to flood the loft space with natural light (Image credit: Simon Maxwell)

Insulating a Loft Conversion

The roof structure can be insulated in one of two main ways:

The most straightforward is to use a ‘cold roof’ method.

This involves filling the space between the rafters with 70mm-thick slab foam insulation, ensuring that there is 50mm spacing between the roofing felt and the insulation (for ventilation via the roof and soffit vents).

In addition, 30mm slab insulation is attached to the inside of the rafters, giving a total of 100mm of insulation. The rafter thickness is often less than 120mm, so a batten may be required along each rafter to allow the 50mm spacing and the 70mm insulation.

The roof section requires 300mm of mineral wool insulation (e.g. Rockwool), or 150mm of slab foam insulation, such as Celotex.

This method uses 100mm Celotex insulation or similar over the rafters, and a covering capping, followed by the tile battens and tiles. This is not really a practical option unless the roof coverings have been stripped off. It could be used with a dormer, especially if it has a flat roof.

Continuity of insulation between walls and roof is required to avoid any cold bridging. The dormer walls can be insulated with 100mm Celotex between the studwork.

The internal partition walls use a 100mm quilt that will provide sound insulation. Plasterboard is attached to one side of the wall then the quilt inserted, followed by plasterboard on the other side.

Insulation is also placed between floor joists, and this is typically 100mm-thick Rockwool fibre or similar — mainly for its sound-reduction properties.

Your Building Control inspector will specify exactly what you require.

(MOREHow to insulate a roof)

Insulating the floor can be achieved by a mineral fibre quilt laid between the joists. Use the heavier, denser sound insulation quilt.

It is often necessary to insulate party walls — both against heat loss and noise. Introducing timber studwork with mineral fibre insulation will allow you to achieve both and it can be covered with sound-rated plasterboard.

Getting Ventilation Right in a Loft Conversion

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.

(MORE: How to solve condensation)

Adding a Bathroom in a Loft Conversion

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.

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
  • 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.

Loft Conversion Fire Safety

Ensure that the new windows are large enough and low enough to escape from:

  • Egress window openings are needed to serve all first floor habitable rooms, but not bathrooms
  • Openings should be at least 450mm x 450mm and at least 0.33m2 in area
  • Rooflights are usually top opening — you must ensure the bottom of the opening is between 800mm and 1,100mm from the floor

Things become more complicated if your loft conversion transforms a two-storey house into a three-storey home:

  • Escape windows that are over 4.5m from ground level are not viable. Instead, the Building Regs require a protected stair enclosure that leads right down to the final exterior door
  • If your staircase rises from a room, rather than a hall, you have two choices:
    1. It can be entirely enclosed within a hallway to an external door
    2. The staircase can be enclosed in a lobby at the base of the stairs. The lobby will have two separate doors, to offer a choice of either a front or back route of escape. These doors and the lobby walls will need to be fire-resistant and most likely open outwards into the rooms to avoid fouling the bottom of the stairs. If the doors do not open outwards into the rooms, they will be acceptable as long as they create viable options for escape in the event of a ground floor fire
  • It can be entirely enclosed within a hallway to an external door
  • The staircase can be enclosed in a lobby at the base of the stairs. The lobby will have two separate doors, to offer a choice of either a front or back route of escape. These doors and the lobby walls will need to be fire-resistant and most likely open outwards into the rooms to avoid fouling the bottom of the stairs. If the doors do not open outwards into the rooms, they will be acceptable as long as they create viable options for escape in the event of a ground floor fire
  • For open plan homes, where the staircase lands in an open plan space, a sprinkler system may be the only option.

The new floor joists of your loft conversion will need to offer at least 30 minutes’ worth of fire protection, which could mean replastering the ceilings in those first floor rooms below.

The loft room will also have to be separated by a fire door, either at the top or bottom of the new stairs.

The existing doors on the stairway to both ground and first floor should be able to provide 20 minutes of fire resistance or be replaced. They can’t be glazed either (unless with fire-rated glass), so you may want to consider windows or rooflights to bring daylight to the stairwell.

Mains-powered smoke alarms should be installed on each floor of your home and interlinked so that they all sound off when one is activated. Most have a rechargeable battery as a back up that allows the supply to be extended from a lighting circuit if necessary.

Wireless, radio-linked alarms can be fitted if you can’t hardwire to the ground floor ceiling.

(MOREPart B of the Building Regulations)