Loft Extension

Going Up! Ten Questions to Consider When Adding a Loft Extension

Are you getting to the stage when you feel your house is creaking at the seams, there’s just too many people and ‘stuff’ and you’re not sure what to do about it? I’m guessing that you’ve thought through all the options, from moving to a bigger place somewhere near, to having a good old fashioned clear out (always a good idea) to seeing if you can jettison one or two family members (not a good idea!). But none of these options appears realistic to you, so you look upwards to the heavens (or at least the ceiling!) for inspiration. And there, right above you, you find the answer and announce to everyone, Loft Extension to create extra space in the home.’

It’s more likely that it is the space requirement rather than the idea of adding value to your property that prompts you to look at adding a loft conversion. But reports estimate that a loft conversion could increase a property’s worth by over £40,000. And if this offers more quality accommodation such as a double bedroom and a bathroom it can add more than 20% to the value of a typical three-bedroom, one-bathroom house.

So, depending on your roof structure, a loft conversion is one of the most straightforward ways of getting extra space. Here are ten questions to consider when adding a loft extension.

What are the different types of Loft Conversions?

There are six main types of loft conversion. In order of complexity (and price), these are:

  1. Rooflight loft conversions – these require the least amount of structural work to the existing space and are therefore the most cost-effective. The only changes required are the addition of windows.
  2. Dormer loft conversions – these are a very popular option as they are the easiest way to add light and an increased amount of roof space with full headroom. The roof structure is altered at the sides or rear of the house to add a large, flat-roofed ‘box’ dormer.
  3. Hip-to-gable loft conversions – these are most commonly found on the side of either end-terrace or semi-detached houses. The hipped (or sloping) side roof is removed and the end wall is then built up straight to form a new vertical gable.
  4. Gable-to-gable loft conversions – these include a new box extension that spans the space between each gable end. It is worth noting that in some cases you’ll need to increase the height of the gable end walls to act as ‘bookends’.
  5. Mansard loft conversions – these can create extra volume and do look very effective on the right kind of house. The process involves the replacement of either one or both roof slopes with very steep sloping sides and a flat roof over the top.
  6. Modular extension loft conversions – these are used where the existing loft space is unsuitable for conversion. The floor space is measured and the new rooms manufactured off-site before being delivered as a module. The existing roof is then removed and the new module installed.

Is My Loft Suitable for Conversion?

When it comes to assessing the feasibility of a loft conversion project, there are a number of factors that will need to be considered. These factors include the available head height, the roof’s pitch and the roof structure, as well as any obstacles such as water tanks or chimney stacks.

Head Height

Measure 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, ask 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 easily conveyed on plans.

The Building Regulations impose no minimum ceiling height for habitable rooms. But don’t forget to consider the stair entry to the loft space. 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.

Pitch Angle

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.

Type of Roof Structure

Two main structures are used for roof construction — namely traditional framed type and truss section type. The traditional framed type is typically found in pre-1960s houses where the rafters and ceiling joists, together with supporting timbers, are cut to size on site and assembled. This type of structure has more structural input, so is often the most suitable type for attic conversions. Space can be easily, and relatively inexpensively, opened up by strengthening the rafters and adding supports as specified by a structural engineer.

Post-1960s, the most popular form of construction used factory-made roof trusses. These utilise thinner – and therefore cheaper timbers – but have structural integrity by the addition of braced diagonal timbers. They allow a house roof to be erected and felted in a day. However, this type of truss suggests that there are no loadbearing structures beneath, and so opening up space requires a greater added structural input.

This will normally involve 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. This added structural input requires skill, knowledge and equipment that would limit scope as far as DIY is concerned — and a far greater cash outlay. It is advisable to seek advice from specialist firms in this instance.

Water Tank

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 (combination) boiler but they do take up a cupboard-sized room, which you will have to find space and budget for.

What Building Regulations must my Loft Conversion Meet?

Do I need Planning Permission to Convert My Loft?

Not normally, no. Loft conversions tend to be considered Permitted Development (as long as the project fits within the specified parameters – check the Planning Portal for specifics). However, if you plan on extending beyond the specified limits and conditions of Permitted Development, then you will need to apply for planning permission.

Do I need Building Regulations Approval?

As a loft conversion is a ‘material change of use’, building regulations must be met. These cover matters of safety, including the strength of the floor, minimum headroom above the staircase, fire escapes, thermal efficiency, electrics, plumbing and glazing. The preparation for building regulations can be completed by either the designer or loft conversion company you are working with.

A building control surveyor will inspect your work at various stages and will issue you with a completion certificate on final inspection.

If your home is semi-detached or terraced, you’ll need to notify your neighbour of your proposals if the works fall under the Party Wall Act requirements. For instance, if you are building in beams which will bear on the party wall(s).

Will I Need New Ceiling Joists?

The existing ceiling joists are unlikely to be adequate to take a conversion floor, so additional new joists will be required to comply with the Building Regulations. The size and grade would have been specified by the structural engineer, who will have taken into account the span and the separation distance for a given loading.

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.

What Loft Insulation Will I Need?

Your Building Control inspector will specify exactly what you require. The roof structure can be insulated in one of two main ways:

Cold Roof Loft Insulation

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 can be undertaken by the DIYer.

Warm Roof Loft Insulation

The other main method is ‘warm roof’. 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.

Insulating the Floor

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

Insulating Party Walls

If you live in a terraced or semi-detached home, it is often necessary to insulate the 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

What Kind of Staircase will I require for my Loft Extension?

The ideal location for a staircase to land is in line with the roof ridge: this will make the 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 roof light above the staircase or, if appropriate, converting a hip roof end to a gable.

The Building Regulations specify that the maximum number of steps in a straight line is 16. This is not normally a problem, as a typical installation usually only requires 13 steps.

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. 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.

The minimum height for Balustrading is 900mm above the pitch line, and any spindles must have a separation distance that a 100mm sphere cannot pass through.

Sometimes a bespoke staircase solution is required. Bespoke staircases cost much more than standard off-the-shelf models, so it pays to consider all your options. It is also worth having the design approved by your Building Control body to ensure the staircase will comply. It is best to email a copy for approval before the staircase is fabricated.

What are the best Windows and Dormer Windows for Loft Conversions?

The loft room will require a means of getting natural light and ventilation.

Rooflights for Loft conversions

Typically, this does not involve much structural alteration. The most straightforward method is to use roof lights that follow the pitch line of the roof. This type is fitted by removing the tiles and battens where the roof light will be fitted. The rafters are cut to make way for the roof light after suitably reinforcing the remaining rafters. The roof light frame is then fitted and flashings added before making good the surrounding tiling.

This type of window is the most economical, and more likely to be allowed without planning permission, under your Permitted Development rights. Conservation roof lights, which are slightly more in line with the roofline and are made of metal, can also be specified.

Dormer Windows

Dormers not only give natural light but can add space to a loft extension; they can be at the ends or sides. 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. They normally involve compound angle cuts so may not be a task that a DIYer would like to undertake. Care also needs to be taken with the roof and side coverings, to get a good weatherproof structure.

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.

Dormers can have gabled or hip roofs, and with careful design can enhance a roof line. In practice, a mixture of the available types can result in the maximum light and space and provide a fire exit.

What do I need to Consider for Loft Conversion Fire Safety?

Single storey

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:

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.

Floor Joists

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 roof lights to bring daylight to the stairwell.

Smoke Alarms

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.

How Much Will my Loft Conversion Cost?

The cost of your loft conversion will depend on your roof structure, the existing available space and whether any alterations need to be made to the floor below to accommodate the staircase.

Room in Roof Loft Conversion

A basic ‘room in roof’ loft conversion is the cheapest and could start at around £15,000. 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 Extension

Another option which does not require dramatic changes to the roof is to do the above and add dormer windows. This will increase the useable floor space and can be used to add head height which gives you more options when it comes to placement of the stairs.

This will cost upwards of £25,000. However, the average dormer loft conversion with a double bedroom and en-suite cost about £35,000–£45,000.

‘Raising the Roof’ or Changing the Roof Structure

This option is the most expensive as it requires the removal and rebuild of the existing roof. It also requires planning permission approval so your local planning permission application cost must be added on.

Another added expense will be the additional design work that may be needed as it is more complicated than a room in the roof, or dormer loft conversion. This type of work is likely to cost upwards of £50,000.

Some companies will create a package type ready-made room which is fabricated off-site and craned into position. The benefit of this is that it is quicker than replacing the rafters and rebuilding the roof thus reducing scaffold hire and labour costs. However, this option costs around £60,000 for the average home.

Will a Loft Conversion Cut My Energy Bills?

Adding a loft conversion provides the perfect opportunity to make your home more energy efficient, as the installation of specific insulation is a requirement of building regulations. There are two forms of insulation and a building control inspector – either an independent inspector or one from your local authority – will determine which type your home requires.

For cold-roof insulation, in total, 10cm of insulation is required; 7cm made up by filling the space between the rafters with foam insulation. The other 3cm of slab insulation is attached to the inside of the rafters. It is essential that a 5cm gap is left between the roof felt and insulation to allow for ventilation.

Warm-roof insulation involves foam insulation 10cm-thick fitted over the rafters before adding capping, tile battens and tiles. This is a more complex option commonly used when a roof covering has been stripped, perhaps when creating a dormer. A 10cm-thick slab of foam insulation can insulate a dormer wall, while a 10cm-thick quilt of insulation is required between plasterboard attached to either side of an internal partition wall. You’ll need 10cm-thick insulation between floor joists.

How Many upheavals will my Loft Conversion Cause?

If you’re having a basic roof light or dormer conversion, then there’s no reason why you can’t stay living in your house throughout the project. Most contractors will erect scaffolding and cover any sections of roof they’re removing with tarpaulin. They will also start with the external work, meaning the majority of your house won’t be affected until they knock through and install the staircase.

If it is a more major project, for instance where most of the roof is being replaced, then you may want to move out for a couple of weeks, particularly if the work is being done during winter. Most loft conversions will take between eight and 12 weeks to complete, while modular lofts can be installed in two weeks.

If you are carrying out loft conversion works and are managing the project yourself you should arrange for conversion insurance to cover the new works and the existing structure. This is because most home insurers will exclude loss or damage whilst the property is undergoing alteration or renovation.

It’s worth discussing your project with a specialist site insurance provider as loft conversion projects can be complex and often include liability assumed under the Party Wall Act 1996.

Site insurance caters for both the existing elements of the property that’s being converted and all the new conversion works that go into the process. The existing structure is usually your house — so if the property collapses while creating a new opening, for example, the renovation insurance will cover it and completely replaces the requirement for buildings insurance, which is not suitable.

All the works, including any temporary works, materials, plant tools and equipment need to be covered. Public liability and employers’ liability is automatically included to ensure you are adequately protected.

How FABRICO can help

FABRICO’s one-stop-shop approach helps our customers to work through the above questions and their implications to determine if a loft conversion is right for them. If they choose to move forward, FABRICO can provide all the necessary guidance and professional support as well as project managing the build itself. We pride ourselves on good communication and ensure that projects run to time and to budget.

We are always pleased to discuss your plans and give you the benefit of our time and expertise. Feel free to contact us either through our website (click here to contact us) or email us