Roof window or dormer for a loft room

A loft is shaped by its roof. Under a pitched roof the headroom is greatest at the ridge and shrinks to nothing at the eaves, so the useful floor is a strip down the middle with sloping edges on both sides. That strip decides what the room can be. A bedroom can live with a low edge where the bed sits, while a bathroom or a study wants standing room where the user actually stands.

Builders commonly work to a clear height of around 2.2m over the main part of the room as a practical starting point for a loft room, though the number is a rule of thumb, not a regulation. If you can reach it over a generous strip, roof windows alone often make the room work. If you only reach it along the ridge itself, or the strip is too narrow for a bed and a walkway, a dormer starts to earn its cost.

Stand in the loft and imagine the furniture. Put the bed head where the roof is lowest and the wardrobe where it is highest. Walk the route to the stairs. If you keep ducking, the room needs volume, and no window will give you that.

A roof window sits in the plane of the roof, between the rafters, with the roof covering flashed round it. It brings daylight and ventilation into the slope without altering the shape of the house. In a loft that already has room to stand, one on each side or two side by side lights the space well. The sky is in the glass and the daylight comes down the slope.

Roof windows suit homes where the outside appearance matters. Slate and tile roofs on Georgian and Victorian terraces in Exeter and the Torbay villas keep their unbroken roofline with a roof window set flush. In a conservation area that can be the difference between a smooth conversation with the planning authority and a long one. Roof windows are also much quicker to install than a dormer, because the structure is a small trimmed opening, not a new room built onto the roof.

They have limits. A roof window adds light, not floor area. It leaves the slope where it was, so the low edges stay low. It also gives you a view upwards and across rather than a vertical wall window, and some people find a loft with only roof windows more enclosed than they expected. Choosing between the two main styles, centre-pivot and top-hung, depends on how high the window sits above the floor, and our guide to top-hung or centre-pivot roof windows covers that.

A dormer is a box that projects from the roof slope, with its own small roof, side cheeks and a vertical window in the front face. The inside gains a patch of full-height ceiling and a vertical wall, which turns the awkward slope into a square corner of usable room. A flat-roofed dormer gives the most extra headroom for the least structure. A gable or hipped dormer looks more traditional and costs more to build.

The point of a dormer is the volume. A bed can sit under a flat ceiling, a bathroom can have a standing shower, and stairs can arrive at a landing with enough headroom. That matters most in a modest loft where the roof is low and the plan is narrow, such as a 1930s semi in Heavitree or a terrace in Newton Abbot.

The costs are real, and not only in money. A dormer needs a structural design, new framing, a new roof of its own, cheeks to clad and insulate, and a window. It takes longer on site, and the house is open to the weather in a way a roof window opening is not. It also changes how the house looks, which brings the planning question.

For a roof window, the relevant permitted development right is Class C. It applies when the window clears the roof plane by 150mm at most and does not climb above the ridge or highest point. A side-facing one has to be obscure glass and fixed shut, unless its opening part is over 1.7m up from the floor.

A dormer is different. It is a roof extension, and it falls under Class B, with its own limits. Broadly, there is a volume allowance, smaller for terraced houses than for others. The dormer cannot go on the roof slope that faces a highway, it must be set back from the eaves, and it cannot rise above the highest part of the roof. Materials are expected to look similar to the existing house. The rights are removed altogether on some designated land, which includes conservation areas and national parks, and householder permitted development simply does not exist for flats and maisonettes. A listed building must obtain listed building consent as well.

That is a summary. The details vary house by house, and an Article 4 direction can remove rights locally. In Exeter, Topsham and Torbay, with their conservation areas, and on the edge of Dartmoor and Exmoor, ask the planning authority before you commit to a design. A roof window is usually the easier thing to agree in those places.

Forming a new roof window opening is building work. The rafters are cut and trimmed, which alters the structure, and the window itself must meet Part L. Where the job is notifiable, Building Control is told by us.

A dormer is a larger piece of building work and always needs Building Control approval. The new structure is designed to carry its own loads and the loads of the roof around it, and the walls, roof and window all have to meet current insulation standards. Turning a loft into a room brings in more than the window. The stairs need adequate headroom, the floor has to carry the new use, and the fire safety of the whole house is looked at. Part B concerns escape from the loft room, and a roof window can serve as an escape window when it meets the size and position rules, so our answer on whether a roof window can be a fire escape sets out what to check.

If you are converting the loft, read our guide to roof windows for loft conversions, which looks at the sizes, positions and numbers that make a room feel right.

Pitch matters a great deal for roof windows. A steeper roof gives more headroom and suits a wider range of windows, while a shallow pitch limits the choice and affects how the window opens and how the flashing is formed. Check the minimum with the window maker, and see our answer on the minimum roof pitch for a roof window.

Structure matters for both. In many Devon houses the roof is cut timber, with purlins and rafters of a size the house's builders chose in their day. Where the trusses of a newer house are used, the web members get in the way of a loft conversion, and of a dormer more than of a roof window. Cob houses across the Culm and Exe valleys may have uneven rafters and heavy thatch or slate loads. Whichever window you choose, someone should look at the timbers before the design is fixed.

This is where the two differ most in daily life, and it is the part we take most care over. A roof window lining runs between the frame and the sloping ceiling, and it is short. Its head sits level and its sill sits upright, with the sides splayed, which lets the daylight move out into the room. The edges are beaded and the paint line is set against the frame.

A dormer has more surface to finish. The cheeks are flat walls, the ceiling is flat or follows the new roof, and the junctions where the old slope meets the new box have to be taped, skimmed and decorated. A dormer ceiling and its joins to the sloping rafters are where cracks show first if the finishing is hurried. Insulation has to be continuous across all those planes or the corners become the cold spots in the room.

If either goes into a plaster ceiling in an older house, the same care applies. Lath and plaster ceilings are supported before they are cut, and the edge is built up in lime where the house is lime. Our guide to how a rooflight is finished inside explains each layer.

We do not quote figures on this page, because the price comes from a survey, but the drivers are easy to name. For a roof window they are the size and model, the roof covering and its flashing kit, the structural trimming, any scaffold needed to reach the slope, and the lining depth and finish inside. For a dormer they are the structural design, the framing, the new roof and its covering, the cladding of the cheeks, the window, scaffold for a longer period, and the plastering and decoration of several surfaces instead of one.

The gap between the two is usually wide, because a dormer is a small building project and a roof window is a fitting. That is not an argument for the cheaper route in every house. A room that cannot be stood in is poor value at any price, so weigh the cost against what the room will be used for.

Yes, and many lofts do. A dormer on the rear slope gives you the standing room, while roof windows on the front slope bring light without changing the street elevation. That split also eases the planning, because the dormer faces away from the highway.

Another approach is to use roof windows to prove the room first. Fit two or three, live in the loft for a while and see whether the volume is enough. A dormer can still be added later, though the roof covering and the finishing will need disturbing again, so it is worth deciding early whether a dormer is likely.

For the room you are likely to have, these questions settle it:

• Can you stand upright over a strip wide enough for the bed and a walkway?
• Does the house stand in a conservation area, or inside a national park?
• Do you want the roofline left alone, or is a dormer in keeping with the street?
• Is the roof deep enough, and the structure sound enough, to open up?
• Do you need a bathroom or stairs with full headroom?

If most answers point to standing room, a dormer is worth designing. If they point to light and a modest conversion, roof windows will do the job well and finish more simply.

We install and replace roof windows, including VELUX, and finish the inside of the opening as carefully as the roof outside. Where a loft room needs a dormer, we are happy to talk through the window and the roof side of it and what it means for the finished ceiling, though the building design itself is something to settle with your designer or builder and the planning authority.

Our quotes are written up with a finish schedule listing the room, what lies above the ceiling, the shape of the lining, its finish and the dust plan. The work carries a 10-year workmanship guarantee. For the whole picture of options and pricing drivers, see the roof window installation page, or ask for a quote and tell us about your loft.