Loft conversions: an essential guide to planning and design to costing

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 provide one of the best returns on investment you can get when it comes to extending, with experts suggesting they add the most value to your home in comparison to the cost. 

Because most loft conversions are generally allowed under permitted development rights, there’s no need to go through the lengthy process of obtaining planning permission. 

Another bonus? You'll get the extra room you need, without having to sacrifice garden space, as is the case a conventional rear or side extension. Find out all you need to know about converting your loft, and check our extensions hub page for more practical advice and inspiration.

bed in a converted chapel with storage built around it

(Image: © Chris Humphreys)

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

The cost will vary depending on size, but is usually between £30,000 and £50,000. A typical conversion with a rear dormer in a mid-terrace property costs around £35,000. You’ll find small companies will usually charge 10-15 per cent less than large companies.

A Certificate of Lawfulness (£86) or planning fees (£172) are often excluded from the agreed contract with the loft company and are paid directly by the homeowner, so need considering.

Building Control Fees (around £500, plus VAT) are also payable by the homeowner to the local authority or a government-approved, independent inspection company, to check that the work is as contracted and to issue building regulation certificates to prove that it has been carried out in accordance.

Expect to pay around:

  • £1,150-£1,350 per square metre for a basic rooflight conversion
  • £1,250-£1,450 per square metre for a dormer conversion
  • £1,350-£1,550 per square metre for complex options

Home office storage with blue cupboards and open shelves

(Image: © Neville Johnson)

How much should you spend on a loft conversion?

There is a limit to how much you should spend on your loft conversion. If you plan on eventually selling the house, you will need to consider the ceiling price of your street.

You want the value of your house to increase by at least the cost of your entire loft conversion, but by spending too much, you may over-value your own house, making it difficult to sell for an appropriate profit.

Comparing the quoted cost of your loft conversion, plus the value of your home, with the cost of moving to a larger house in the same area is a worthwhile practice in assessing the benefits of converting the space.

For example:

Your house is valued at £270,000.

The loft conversion costs £50,000, making the projected cost of the home £320,000.

However, the ceiling price of your street is £300,000, making it impossible to recoup the £20,000 excess spent on a loft conversion.

If there is a house in your area with the space you require for less than £320,000, it is worth considering a move, rather than investing in the loft conversion.

Does a loft conversion need planning permission?

Does your loft need planning permission? The answer is: not usually, but always check with your local planning department. As a general rule, loft conversions are classed as permitted development and generally 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

Building regulations for loft conversions

Always remember that planning permission does not equal building regulations approval – the two have to be cleared separately. Every new conversion, including those done under permitted development, must comply with fire and building regulations. This covers the safety and quality of the building work, including:

  • checking the proposed structure is calculated properly;
  • the safety of the stairs;
  • insulation levels;
  • effective drainage;
  • electrical safety.

Ask your local authority’s building control department, or a private sector approved inspector, to help early in the planning stages. A common pitfall is the need for a ‘direct means of escape’, so a separated stairway and hallway with fire-rated doors to all rooms, will be required (see below).

Loft bedroom with en suite in a 15th century converted mill

(Image: © Solebury & Worthy)

Fire safety in a loft conversion

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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.

Building regulations require loft conversions that are 4.5m above external ground level to have 30-minute fire protection in the floor and walls, including the protected route to an external door. This often means upgrading doors to all habitable rooms on to the stairwell to fire doors. In a bungalow, a fire escape window with a clear opening of at least 45cm x 45cm in each habitable room is acceptable. In an open-plan house, sprinklers may be a suitable alternative.

At least one mains-operated smoke alarm with battery back-up must be installed in the circulation space of each storey.

What if your stairs rise from the living room, not a hallway. Is this permitted?

No, in this case, the stairway must be separated from the rooms by walls and fire doors, leading all the way to an external door, and not open-plan to rooms.

You could create a hallway by partitioning off the stairs from the room, or if the stairs are alongside a spinal wall separating two ground-floor rooms, you could form a lobby at the bottom of the stairs with fire doors leading to each room. So long as you have separate escape routes from both of these rooms, it would be acceptable.

If you really want an open-plan layout to include the stairs, sprinklers are usually the only option.

When converting a loft in a single-storey home, it is permissible to have the stairs within a room as you don’t need a protected stairway. In this situation, the requirement for an alternative emergency fire escape could be met by a first-floor window.

Is my loft suitable for conversion?

‘Most properties will be suitable for a loft conversion so long as they have a loft that measures 2.3m 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 when converting a loft

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 your loft 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.

(Image: © Chris Snook)

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 10 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

Maximising light in a loft conversion

Loft room windows – and doors for that matter – can make a vast difference to the appeal of your space. If they can’t be opened, you can be quite ambitious about their size – a loft conversion company or architect can advise on this to help you get your plans approved by the local council. You can also choose bespoke or ready-made French or folding-sliding doors that can open out onto roof gardens or balconies with glazed balustrades for uninterrupted views.

If your budget is too tight or if planning is an issue for large expanses of bespoke glazing, you can cleverly maximise the impact of ready-to-fit rooflights by grouping them to create a wall of glazing for a fraction of the price. For the best views, choose top-hung windows, and use its proportions to make the room seem larger. For example, windows spaced evenly along one section of roof can make a long, narrow room feel wider; windows in a tight group of four can make a low ceiling feel lofty; or, for a small space, consider Velux’s Cabrio, which can open right up to create an instant balcony, making the room inside feel much larger. Bear in mind that the position of windows will be dictated by the shape of your roof, and always get your plans approved by the local council before you start work.

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.

If you have space up top, use it to create extra rooms, or a stunning master suite

(Image: © Clear Architects)

How to choose a staircase for a loft conversion

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.

Gable-end windows

Most loft conversions will have at least one vertical external wall and standard window openings can be formed in these walls to bring in extra light. New windows in side elevations do not usually require planning permission if they are obscured, or are more than 1.7m above floor level.

Heating and ventilating a loft conversion 

Before work begins, check your boiler can cope with the extra demand for heating and hot water. In particular make sure the water pressure is sufficient – you might need to raise the height of the header tank, or switch to a pressurised plumbing system. 

Ensure good natural airflow by placing windows that open at opposite ends of the new room, as loft rooms can get warm.

Designing a loft conversion

Key considerations:

  • Is it the best use of the space?
  • Is there space for furniture and storage?
  • Is the ceiling height acceptable?
  • Could the addition of a dormer create more space?
  • Does the position of the stairs optimise potential space?
  • Could more light be created with roof lights or light tunnels?

When an architect designs your loft, they will generally look for the most cost-effective option that will have least impact upon the rest of the house. However, as long as the construction is structurally sound and the work meets building regulations, there is no harm in exploring alternative placements for stairs and doors.

Usually, the most efficient position will be above the staircase that links the ground floor and first floor, but the best position will depend upon available heights, proposed use, roof shape and the position of the door to the loft room.

When the proposed space is one room, it is often good to locate the door to the space at the foot of the staircase rather than at the top to create an additional sense of space and openness on the new floor. You could also locate the door halfway up a new staircase with a double landing, as you can’t have a door right on a staircase. This gives an open feel at both the top and bottom of the staircase.

Converting your loft may result in the loss of space at first-floor level to accommodate the new staircase. Losing a small bedroom on the first floor to provide a larger one on the second floor is rarely a good idea, but may be worth considering if, for instance, part of the small, compromised first-floor bedroom can be used as a new en suite or dressing room to the master bedroom.

It is always worth running your plans and designs past an architect or designer that can visualise them with 3D drawings. The issue with a loft conversion is that floor plans can look large, but don’t take into account the loss of usable space due to sloping ceilings.

Digital models can still be misleading depending on the viewpoint of the particular software. Often a simple paper model will create the best impression of how the space will actually function. Keep in mind factoring the cost of modelling into your design budget to avoid disappointment when the loft is completed.

Choosing a loft conversion company

When choosing a specialist loft company, you will benefit from an integrated project team who all work together to ensure the journey from initial concept to final build is as simple and stress-free as possible. The loft company will provide professional architectural designers, qualified Royal Institution of Chartered Surveyors (RICS) structural engineers, surveyors, loft estimators, project managers and specialist tradespeople and loft teams. This structure means that the company is able to see the project as a whole from the outset, giving it the ability to foresee potential problems.

Do not be persuaded to simply opt for the cheapest company. Find a company or person that makes you feel comfortable, and always ask to speak with a previous customer and whether you can pop over to have a look at the work done. Admire the basics, such as how a hinge has been chipped into a new door, or the quality of the new roof and appearance of the new dormer. Find out if their chosen company went the extra mile by asking questions. Were they punctual, clean and tidy throughout the build? Did they start and finish the loft conversion within an agreed build schedule? Did they ask for any extra money along the way or was it a fixed price? You will find a lot of loft companies happy to commit to a fully fixed price and to sign documents to reflect this. As they are the professionals, it is their duty to capture all the costs, not yours.

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