Regent Street Dormer - by Konishi Gaffney Architects - photography by Alan Craigie
Utilising redundant space in your roof is often an excellent way of creating additional living space in your home.
When compared to other types of home extension, dormer loft conversions often work out more economical to build as they convert existing space rather than create space from scratch. No need for expensive foundations or groundworks, and a lot of the space is already there to be exploited.
Loft conversions tend to work best in older properties which typically come with large attic spaces, but they can be built onto modern houses with clever engineering – more on this below.
Where a standard loft conversion simply converts an existing roof space into habitable floor area, a dormer loft conversion includes the addition of a box shaped structure – the roof dormer – which increases the head height and provides more useable floor area.
For the most part, the benefits far outweigh the negatives.
The advantages of adding a dormer on a roof include:
The disadvantages:
Roof dormers come in a wide range of shapes and sizes. The key to a successful loft conversion project is designing a dormer that strikes the right balance between internal space and external appearance.
If you are building in a heritage setting, such as a Conservation Area, local planning policy will likely dictate the size and appearance of your roof proposals.
Below are the main types of roof dormer built in the UK.
Flat roof dormer
Typically built full width on the rear roof slope of existing homes under permitted development, the box-like profile of a flat roof dormer will guarantee you achieve the maximum internal useable space possible. The side cheek walls and rear facing wall are vertical, and the roof is built to a nominal fall. Can be visually unattractive if poorly designed.
L-shaped dormer
Applicable mainly to Victorian terraced housing, an L-shaped dormer usually takes the form of a flat roof dormer that returns part of the way over the existing ‘outrigger’ at the rear of the house to create an enlarged usable roof space.
L-shaped dormers seek to maximise the available roof volume prescribed under permitted development. However, some local councils consider L-shaped dormers to fall outside of permitted development rights, and it is advisable to apply for a certificate of lawfulness before starting work on site to avoid potential problems with planning enforcement.
Mansard dormer
Like a flat roof dormer but with a sloping rear wall (traditionally built at around 70 degrees), a rear mansard dormer offers a more sympathetic appearance at the expense of slightly less floor area. Mansard dormers can be built in an L-shape configuration to maximise floorspace, subject to local interpretation of permitted development rules as noted above.
Gable dormer
The remaining roof dormers on this list typically form individual dormer windows rather than full width roof additions, as noted above. These are usually incorporated into dormer loft conversions located in heritage settings where larger dormers are restricted under local planning.
A gable dormer is a dormer with a dual pitched roof and a flat facing gable end. A common architectural feature in older properties where the roof is used for habitable space.
Hipped dormer
Like a gable dormer but with a front sloping portion of roof. The roof geometry is slightly more difficult to build than a gable dormer and requires guttering on all three sides. An appropriate choice if matching in with existing dormers or to suit the architectural style of the host building.
Shed dormer
A shed dormer is effectively a small, flat roof dormer with a steeper roof pitch. Frequently incorporated into mansard roof extensions to create a pleasing overall composition.
Wall dormer
Also sometimes referred to as a lucarne, a wall dormer is where the dormer breaks the eaves line of the main roof and merges into an external wall below. A common feature in Georgian domestic architecture where the main roof forms part of the upper storey. Wall dormers often feature in contemporary buildings that draw inspiration from traditional building forms.
Dormer extensions do not need to be finished in standard plain concrete tiles and uPVC glazing. There are so many more options. Hiring an architect will open a huge range of design possibilities and elevate your dormer extension to another level.
When it comes to the design of the glazing elements, consider the wider context of the site.
If your home backs onto an open view, consider a large picture window or Juliet balcony to frame the view. Installing a blind or curtain with a recessed track hidden away in the ceiling will allow you to control privacy and preserve the quality when the curtains or blinds are drawn.
If you live in urban area where houses closely back onto each other, it’s advisable to minimise the amount of glazing at the rear to ensure good levels of privacy and install larger rooflights to offset any reduction in natural light. Being in a loft presents you with an opportunity to frame a view of the sky – way more attractive than the backs of terraced housing!
As loft glazing is usually quite a reach from ground level, it can be difficult to clean and maintain. Installing reversible opening windows and rooflights will allow safe cleaning from the inside. Fixed glazing should only be considered where good external access can be provided.
Lofts can get very warm in the summer. Consider heat reflective glass and minimise the amount of south facing glazing to reduce solar gain in the summer months. Installing blinds, and even external shading devices, will also help keep the heat out and inside temperatures cool. Opening windows in the loft and floors below at night will create a stack effect and help expel hot air rising through the building.
For the external materials, a good quality roof tile is always a safe bet. If you are converting a Victorian or Edwardian roof space, consider a dark grey Eternit concrete tile if natural slate is too expensive. Combined with dark grey windows and rainwater goods, you will achieve a high-quality, stealth-like aesthetic.
Alternatively, consider metal sheet cladding, timber or fibre cement board for a more contemporary appearance. All three materials can be specified so the colouration is like your existing roof materials – a requirement if you are seeking to build under permitted development.
Some local planning authorities take a more conservative approach to design than others. An architect will be able to advise on what is possible in your area.
The design of the new loft staircase is very important to a successful roof conversion project.
It is normally best to continue the existing stair configuration up to the new loft level to keep circulation to a minimum and maximise the availability of useful floorspace. More on this below.
If you live in an older property with an attractive existing staircase, a common approach is to try to make the new stair look like the existing one. In practice, however, this can be very difficult to do. The configuration of your loft stair is unlikely to be the same as the flights below which makes it almost impossible to replicate the design.
In many cases it’s often best to embrace the fact that the new stair is better off being different and install a contemporary design to contrast with the old. Building a contemporary stair can mark the end of your old house and the start of something altogether different above.
More:
Loft conversion stairs - architect advice and design tips
When it comes to the design of the interior, you have scope to do something different to the rest of the house as the spaces are largely self-contained.
The angled surfaces of a roof space make it hard work for plasterers. Instead consider lining the walls and ceiling with high-quality plywood or tongue and groove boards to create a warm and inviting interior space.
A good-quality engineered timber floor or natural wool carpet are always a safe bet for the floor. As an alternative consider cork flooring. Cork is highly sustainable, feels warm under foot and offer excellent sound absorption. See our article on cork flooring - An architect's guide to cork flooring.
If space is tight and a standard hinged door will interrupt the flow of the space and clash with furniture, install pocket sliding doors. They are a little more expensive but will maximise the spatial potential of your loft room.
The sloping ceilings can present a challenge to lighting design. If you have a low floor to ceiling height – which is often the case – consider wall and floor lights over recessed down lights, as these can feel oppressive in a low space. Hang pendant fittings within higher volumes such as voids over stairs.
Converting a loft into useable floorspace is dependent on several factors, so it’s important to consider the following.
Existing roof height
The main thing to check to see if your property is suitable for a dormer conversion is your existing roof height. Measure the distance from the top of the ceiling joists to the underside of the ridge beam at the apex of the roof.
The ridge height determines everything as it sets out the maximum height of the dormer. Properties with a low ridge height are not always suitable for conversion as building a dormer will not make up for the lack of height.
The optimum ridge height for a dormer loft conversion is 2.7 metres, which will achieve a finished dormer ceiling height of about 2.4 metres, factoring in a new supporting floor, steelwork, insulation and interior finishes.
The minimum roof height for conversion is about 2.4 metres, which will achieve a finished dormer ceiling height of around 2.1 metres. There is no minimum room height requirement in the UK; however, anything below 2 metres will feel very uncomfortable. Further, building regulations require a minimum clearance of 2 metres above stairs, reducing to 1.8 metres in certain situations. If you can’t provide this head room, your project will not gain approval.
If your attic space is not tall enough, you have two options. One, raise the roof. While structurally feasible, it may be difficult to obtain planning permission, especially for a terraced or semi-detached house. Two, lower the ceilings in the rooms below if you have sufficient space to do so. Both options will result in higher build costs and take longer on site.
Find more information on standard UK ceiling heights here:
What's the UK's standard ceiling height for houses, extensions and loft conversions?
Existing roof structure
Your existing roof structure will have an impact on suitability of conversion.
Domestic roofs are usually constructed from either traditional rafters and purlins or truss rafters depending on when it was built.
Roofs formed in traditional rafters and purlins – also known as a ‘cut roof’ – feature in older properties (pre-1960s) and leave most of the space within the roof void clear. Modern roofs are often built with truss rafters, which are formed from lots of small sections of timber and grouped closely together, and take up a lot of roof space inside the loft.
Older properties are therefore usually much easier to convert and typically require simple structural works to make the space suitable for habitation.
Converting a modern roof made up of truss rafters is possible but extra structural support is required. The favoured approach today is to install TeleBeams, lightweight telescopic aluminium beams, and new timber rafters alongside the existing roof structure. This allows the obstructing diagonal sections to be cut away to form a clear void.
It's important to seek advice from a structural engineer early in the design process to understand what is feasible.
Stair access
The location of your existing staircase will have a significant bearing on overall costs and the suitability of your loft space for conversion.
The practical solution for loft access is to continue the existing staircase up to the new loft floor. If the continuation of the existing staircase arrives at a point that hinders the proposed layout - for example, the stair arrives in the middle of where you want to position the bedroom - then you may have to consider relocating the existing staircase at considerable cost.
Dormer loft conversions work well in Victorian terraced housing because the stairs are usually positioned parallel to a party wall, leaving a large area in the loft for living space.
In Edwardian housing, however, the staircase sometimes runs side to side across the centre of the floor plan. If you were to continue this configuration up to the loft level, the new stair would arrive in the middle of the plan, resulting in a hugely compromised living space. In such a scenario, the only solution is to install a new staircase from the ground up and adjust the entire house layout to suit.
Site location
Where your property is located may determine the size of the dormer and therefore the suitability of the roof for conversion. For example, dormer loft conversions to houses in conservation areas are normally restricted to small individual dormer windows as they do not enjoy the same permitted development rights as other properties. The resulting restriction on usable floor space may determine whether a dormer conversion is feasible for your requirements.
Plumbing and services
You may have heating equipment, such as a water tank, in your attic space which will need relocating or removing.
If you are proposing to include a new bathroom in the loft space, you’ll need to ensure there is adequate drainage nearby to ensure the sanitary ware can be designed and positioned as you want it. If existing provisions are not adequate, you’ll need to install new drainage connections which may cause disturbance to the lower levels of your property.
In many cases, roof dormers to houses can be built under permitted development, meaning you don’t need planning permission to carry out the works.
If your property is listed or in a protected planning area, for example a Conservation Area, National Park or Area of Outstanding Natural Beauty, then it’s likely you will require planning permission.
A dormer roof extension to a flat or maisonette will require planning permission as these types of properties do not have permitted development rights.
If you can build under permitted development, you may still wish to apply for a certificate of lawfulness to ensure your proposals meet design guidelines.
Permitted development rights allow up to 40 cubic metres of additional roof volume for a terraced house and 50 cubic metres for a semi-detached or detached house.
The height of the dormer cannot exceed the existing ridge height of the house and the rear wall needs to be inset 0.2 metres for the roof eaves.
Proposed materials must be similar in appearance to the existing house.
Find more information on permitted development here.
Permitted development rules for house extensions
All dormer loft conversions require building regulations approval, which ensure the works are safe for habitation and meet current technical standards.
Simple loft conversion works can be built on a Building Notice where the design and building work is signed off by the building inspector as the project progress on site. Alternatively, if your dormer extension is complicated, or you want peace of mind that the design meets technical requirements, you can get the plans signed off beforehand, leaving just the building work to be approved on site.
If your loft works are to a semi-detached or terraced house, it’s likely you will need to form a party wall agreement with your neighbour(s) before starting work on site.
A simple party wall agreement can take a few days to resolve. However, if a neighbour dissents to the work, it can take a lot longer – in some cases several weeks.
Appointing professionals, such as an architect, structural engineer and party wall surveyor, will ensure your project runs smoothly. They will produce drawings and calculations to demonstrate your proposal meets with the latest regulations, as well as obtain the necessary statutory approvals.
As is the case with all types of building work, build costs are dependent on a range of factors including size, design complexity and specification.
As a rough guide, a typical full width, flat roof dormer loft conversion will start from about £35,000 plus VAT. Adding an ensuite bathroom will cost a further £5,000-£7,000 if built to a simple specification.
L-shaped dormers to terraced housing start from about £50,000.
A full width mansard style dormer extension will cost about £45,000, increasing to about £65,000 for an L-shaped version.
In addition to build costs, you will need to allow for professional fees and statutory approvals.
A measured building survey will cost from £700.
If you wish to appoint an architect to design your dormer loft conversion, fees are likely to start from £2,000 plus VAT up to planning stage, and will cover general design work, planning advice and the submission of your planning or lawful certificate application.
Architect fees for building regulations approval are likely to start at around £3,000 plus VAT.
A structural engineer will charge about £800-£1,000 to provide structural drawings and calculations for building regulations approval.
A Certificate of Lawfulness application costs £103 in England, £300 in Scotland and £85 in Wales, and take up to 8 weeks to process.
Should you require planning permission, application fees cost £206 in England, £300 in Scotland (or £225 if in a conservation area) and £190 in Wales.
A typical average cost for a party wall agreement with your neighbour will cost around £1,000 plus VAT.
Building control fees will cost around £700 plus VAT, and are payable to the local authority or a government approved private building control company.
See the article below for more inspiration.
23 loft conversions designed by architects
About the author: Aron Coates is an architect with over two decades of experience, encompassing projects ranging from the restoration of historic buildings to the design of contemporary homes.
The information provided in this article is intended for general guidance and educational purposes only. At Designs in Detail, we advise you appoint a skilled residential architect to provide specific expert advice for your project.
