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Loft Conversions: The Go-to Guide for Converting Your Roof Space

a split level loft conversion
(Image credit: Sean and Stephen Architects)

Extending up into a loft conversion is a great way to add an extra bedroom, home office or even squeeze another bathroom into your home. When it comes to creating more liveable space and adding value, a loft conversion is a bit of a no-brainer when you're in need of extra room. 

However, there's plenty to consider when approaching a loft conversion project. It can be a large project, with the most usable types of loft conversions requiring not only the existing space to be converted, but extensions added to the roof to increase the available headroom. 

And while loft conversions can be cost-effective ways to add space compared to extending, that doesn't necessarily mean they're cheap. 

That's why it's important to get it right. With our in-depth guide, written by the experts, we cover the likes of Building Regulations, planning permission, costs and the execution of your design to ensure you get the very best out of your space. 

Is my Home Suitable for a Loft Conversion?

Not all roofs are suitable for a loft conversion, while some roof types will require extra structural work or a certain type of loft conversion to meet requirements. 

The main considerations as to your home's suitability are:

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

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. Typically found in pre-1960s houses, rafters on traditional roofs run along its edges, leaving a good amount of free space. The rafters may need to be strengthened or additional supports added (your structural engineer will advise on what is required).

attic truss roof

(Image credit: iStock)

Trussed roofs have ‘W’ shaped rafters which support the roof and the floor structure. Truss roofs are harder to convert, but not impossible; the ‘W’ shaped rafters can be replaced with ‘A’ shape structure which creates a hollow space. This normally involves 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.

While this can add additional costs, it could be a worthy investment, so take this into consideration during your planning process.

(MORE: Loft Conversion Ideas)

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.

How Much Head Height is Required 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.

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.

(MORELoft Conversion for Difficult Roof Constructions)

a roof with a steep pitch provides ample room for a loft conversion

Properties with a steep roof pitch provide the opportunity for a striking loft conversion. (Image credit: Getty Images)

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.

Can my Home’s Foundations Handle the Extra Weight of a Loft Conversion?

Adding the extra weight of a loft conversion to an existing home can put extra stress on the foundations, and in some instances cause subsidence. Your home's foundations should be checked before carrying out a loft conversion — to do this, your builder will need to dig a trial hole to expose the foundations. Your building inspector may want to check these also. 

If your foundations are deemed not deep enough to support the extra weight, you'll likely require a structural engineer's input. They may suggest structural interventions to spread the load of the weight through structural beams, or else look at underpinning the foundations. Both of these can add a lot to the cost of your project. 


How Much Does a Loft Conversion Cost?

What type of homes are cheapest for loft conversions?

Bungalows are generally the cheapest properties to convert because most (but not all) have super-spacious lofts. The taller the building the greater the cost due to all the extra fire-safety work and additional lifts of scaffolding.

Loft conversion costs will be significantly lower where you’ve got a suitably spacious loft to start with. 

A generous height is the most important factor, and if the existing floor area is also a reasonable size it should make it easier to install large beams and loft stairs without major surgery. 

It also helps if your roof has gable end walls rather than sloping hipped roofs to the sides. And traditional timber roof structures are generally simpler to convert than more modern (post 1970) trussed rafters with dense W-shaped webbing obstructing the roof space.   

Loft conversion costs can vary from around £18,000 to as high as £65,000.

  • A basic loft conversion would be likely to cost from around £18k to £25k for a straightforward double bedroom design
  • Adding large dormer windows and an en-suite bathroom can raise the total to around £40k
  • Most expensive of all, a full rebuild with a totally new roof could total as much as £65k

a loft conversion built behind a house's brick gable

In this project by Inglis Badrashi Loddo, a new rear-facing loft extension was built up behind the brick gable end of the existing roof, providing space for a new children's floor.  (Image credit: Brotherton Lock)

The biggest savings are likely to arise where you’ve already got a spacious loft which doesn’t require the addition of bulky dormer windows, just some rooflights fitted flush with the roof. 

Online estimator websites can be rather hit and miss because they’re a bit simplistic but some quote prices per square metre (for the existing loft floor area) which are typically:

  • Simple ‘skylight’ loft conversion: £1,000 to £1,350 per m2
  • Dormer loft conversion: £1,400 to £1,900 per m2

Getting prices from specialist contractors is normally the best way to get an accurate idea of your loft conversion costs. Normally you’d want to get three or four separate quotes based on a clear description of the new accommodation you want.  

Extra Costs for Loft Conversions

It’s not unusual for contractors’ quotations to discreetly omit works such as decoration, supplying bathroom fittings, and they can also be a bit vague about design details such as Juliet balconies. 

Also VAT sometimes isn’t included in the figures. So before placing too much faith in bottom line prices make sure you’re comparing like with like.  

Another area to remember are the rules and regulations and their associated fees: 

  • Architects fees: £1,200 to £2,400 for planning drawings
  • Planning permission application (if required): £300
  • Building Regulations application: £400 (not including drawings)
  • Structural engineer calculations: £1,000
  • Party Wall Surveyor (in semis & terraces): allow £900 per neighbour

Will a Loft Conversion Add Value to my Home?

a bathroom in a loft conversion

A loft conversion can be an effective way to include an extra bathroom in your property, but you'll need to consider the location of the existing services.  (Image credit: West One Bathrooms)

Research by Nationwide conducted several years ago found that a loft conversion which includes a bedroom and bathroom could add as much as 20% to the value of an average three-bedroom, one-bathroom property. 

With the average UK house price currently £256,405, this means that the average loft conversion could add over £51,000 to the value of a property: however, this is an approximate guide, and will depend on a multitude of factors, including the ceiling price for your area. If you’re concerned about the return on undertaking a loft conversion, ask an estate agent for advice before undertaking your project. 

(MORE: How to Add Value to Your Home

How Long Does a Loft Conversion Take?

The length of a loft conversion project depends on the complexity of the design, and whether any structural reinforcements are required. However, on average, a loft conversion project can be completed in between 6-10 weeks, with around 8 weeks the average time. 

Check out this example of a schedule of works for a loft conversion to see how the time is allotted. 

Do I Need Planning Permission for a Loft Conversion?

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)
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Join The Planning Hub

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You won't always need planning permission for a loft conversion, as in most cases, they tend to fall under Permitted Development (PD) rights. 

However, your design will need to adhere to a number of specified parameters, the most relevant of which include:

  • The property must not have already used its Permitted Development rights to add an additional storey.
  • It must not add volume of more than 40 cubic metres for terraced houses or 50 cubic metres for all others to the existing roof space.
  • A loft conversion must not exceed the height of the existing roof under PD rights.
  • It also can’t extend beyond the existing roof slope on the house’s principal elevation (where it fronts onto a highway).
  • Verandas and balconies with raised platforms are not allowed under Permitted Development, but Juliet balconies are.

a loft conversion with a balcony is not permitted under permitted development

Adding a balcony to a loft conversion means it will require planning permission.  (Image credit: Simply Loft)

There are also parameters during the build on interaction with the existing eaves and the location of windows.

Loft conversions are also not considered Permitted Development in designated areas, such as conservation areas and Areas of Outstanding Natural Beauty. In these instances, you would be required to apply for Planning Permission 

In some areas, permitted development for loft conversions may have been removed, so you always need to check before undertaking a conversion project. 

Loft Conversions and the Building Regs

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.

Loft conversion by Simply Lofts

Bespoke, built-in storage a great way to ensure you make efficient use of every inch of space (Image credit: Simply Lofts)

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 (more on that later in the article).

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 bedroom

Loft conversions provide a great opportunity for creating a new bedroom. (Image credit: Chris Snook)

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.

In practice, the actual position of your loft conversion stairs 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.

Do I Need to Change the Ceiling Joists?

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.

Choosing Windows for 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.

Dormer Windows

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.

loft conversion bedroom with bespoke built in storage

Many homeowners want to convert their loft in order to create a new master-suite or guest bedroom. Built-in storage is essential to make best use of the space. (Image credit: getty images)

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 bifold 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 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.

a loft conversion bedroom with a freestanding bath

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

The Warm Roof Method

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.

Insulating the Floor

Insulating the floor can be achieved by a mineral fibre quilt laid between the joists. 

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.

Your Building Control inspector will specify exactly what you require.

(MOREHow to insulate a roof)

Do I Need to Soundproof my Loft Conversion?

Yes, you should consider soundproofing elements during a loft conversion. This can be easily incorporated when insulating floors and party walls by choosing the heavier, denser sound insulation quilt.

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 a loft conversion with dormer window

Adding a dormer window during your loft conversion can provide extra head height where you need it. (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.

(MORE: Loft Conversion Bathrooms)

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

This ensuite bathroom was created in a 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.

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Michael Holmes

Michael is HB&R’s Head of Content and Product Development. Michael is also, Chair of the National Custom and Self Build Association (NaCSBA), presenter of multiple property TV shows and author of Renovating for Profit (Ebury). Michael is a regular in the seminar theatres and Advice Centre at the Homebuilding & Renovating Show.