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Loft Conversion Guide: How to Plan a Successful Project

Open plan living area with access to terrace in loft conversion
(Image credit: getty images)

If you need to create additional space at home, a loft conversion is well worth considering, especially as it will make better of use of existing space. But it is a project that requires careful thought and planning. 

When converting your loft, you will not only have to factor in the type of roof you have (some roof types are easier to convert than others) and how you will use the additional space, but you will also need to make sure you adhere to strict planning rules and Building Regs stipulations. 

You will also need to think carefully about how you will insulate and heat the new room, how you will bring in natural light and, crucially, how you will access your converted loft. 

Expect to pay anywhere from approximately £18,000 for a basic room-in-the-roof loft conversion, to around £90,000+ for a modular loft conversion which is manufactured off-site then craned into place.

This guide will take you through every step of the process so you can tackle your loft conversion project with confidence.

(MORE: Get a tailored quote for your loft conversion)

Is a Loft Conversion Worth It?

Loft Extension in Victorian London Flat

Full mansard roof loft conversion suit terraced houses well — adding maximum space for homes with a limited overall footprint.  (Image credit: Simon Maxwell)

Often less expensive than moving house, a loft conversion makes better use of existing, yet underused, square footage in your home and, in most cases, makes more financial sense than building an extension (and sacrificing a portion of your garden in the process).

Nationwide has estimated that a loft conversion featuring an additional bedroom and bathroom could add around 20% to the value of a three-bed, one bathroom house (but do make sure you factor in price ceilings in your area). 

A loft conversion is especially suited to those properties in tight urban areas where space is a premium, or where a conventional two-storey extension doesn’t make financial sense (or isn't permitted under local planning rules).

Do I Need Planning Permission for a Loft Conversion?

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).

(MORE: Loft Conversion Ideas)

Dormer loft conversion

A simple dormer loft conversion such as this will not usually require planning permission — unless you are in a conservation areas or live in a listed building.  (Image credit: Simon Maxwell)

How Much Does a Loft Conversion Cost?

Loft conversion costs vary hugely depending on a number of factors, including the type of loft conversion you are carrying out, your location, what you plan on using the loft for and your existing roof structure. 

Plan for a starting from figure close to £35,000 to £40,000 and bear in mind that many loft conversions end up costing hundreds of thousands. 

Sticking to off-the-shelf products (rather than bespoke) and being cautious about the spec of chosen fixtures and finishes can help to keep costs under control. 

(MORE: Types of Loft Conversion)

Double bedroom in loft conversion

A loft conversion can be a great idea if you are looking to add an extra bedroom to your home (Image credit: getty images)

Editor's Note: Homebuilding.co.uk partners with the UK's best loft conversion specialists to match your requirements with their services. Simply answer a few questions on what you need from your loft conversion and we’ll put you in touch with a suitable partner.

Do I Need Building Regulations Approval for a Loft Conversion?

Building Regulations approval will always be required when converting a loft. A building control surveyor will come to site to 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.

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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 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. 

Storage from Loft Leg

If you only plan on using your loft for storage as opposed to a habitable space, a simple rooflight conversion will usually suffice.  (Image credit: Loft Leg)

Is my Loft Suitable for Conversion?

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 available – but costly – solutions that will require professional input.

(MOREHow 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.

Bedroom and work space in loft conversion

A loft conversion, done well, can add up to 20% to the value of your home (Image credit: getty images)

Do I Need an Architect to Design 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.

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)

loft conversion

Loft conversions provide a great opportunity for creating a new home office. Ensure rooflights have good blinds, such as these from Blinds2go to prevent glare on computer screens.  (Image credit: Blinds2go)

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. 

How to Add 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)

Will Ceiling Joists Need Replacing?

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.

What Windows Should I Use in a Loft Conversion?

window treatments for rooflights in a loft conversion

Rooflights are a great choice for bringing natural light into your converted loft (Image credit: Blinds 2go)

There are two main 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 of window is the most economic, and more likely to be allowed without planning permission.

What is a Good Loft Conversion Lighting Scheme?

As with any interior scheme, the best way to design your lighting is by using a combination of different light sources, 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.

How to Heat 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: 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)

How Should I Insulate my Loft Conversion?

There are two main way by which you can insulate your roof. 

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.

Loft conversion

For a loft conversion to be compliant with Building Regulations, good insulation is essential (Image credit: getty images)

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.

What Ventilation Will my Loft Conversion Need?

In order 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)

Can I Add a Bathroom in a Loft Conversion?

bathroom in loft conversion

Be mindful of where your ceilings will slope in a loft conversion when it comes to planning the layout of your furniture. Placing a bathtub under a sloping ceiling can be a great use of an awkward spa (Image credit: getty images)

Many people choose to include a bathroom within their loft conversion — and it is really something of a must if you plan on locating a bedroom up there. 

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.

En suite bathroom added during loft conversion

En suite bathroom in loft conversion by Simply Loft  (Image credit: Simply Loft)

Loft Conversion Fire Safety

It is essential to follow the Building Regulations concerning fire safety measures when it comes to converting your loft. 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)