Choosing between a dormer and a rooflight usually comes down to two things: how much extra headroom your existing roof gives you, and how much you want the roofline to change from the street. A dormer adds standing height and floor area by altering the roof structure; a rooflight works within the roof you already have, which usually means a shorter build and a lighter planning process — provided there's enough head height to start with.
Dormers and rooflights solve the same underlying problem — a loft that's too dark, too low, or both — but they do it in very different ways, with different visual impact, planning implications and cost. Recommending one over the other isn't a matter of preference; it depends on what the existing roof structure and shape will actually support, and how much change to the roofline is acceptable or desired. Neither option is automatically better — a rooflight that leaves you short of usable floor space isn't a saving if you end up wanting a second, larger project later, and a dormer on a roof that didn't need one adds cost and disruption without a matching benefit. We size the recommendation to what the room will actually be used for.
The right choice depends on how much extra headroom the space actually needs, how visible a change to the roofline is from the street or neighbouring properties, and what the existing roof construction allows without disproportionate structural work.
A practical way to add headroom and useful floor space to many London roof forms.
A rear dormer typically has fewer planning constraints than alterations visible from the street, since it usually faces away from public view — this makes it one of the more straightforward routes to genuine extra headroom on many London roof forms. The size and position are still governed by permitted development limits or planning conditions, which we check as part of the assessment.
Potential additional space over rear additions where the property and permissions allow.
Where a property already has a rear addition (a kitchen extension, for example), an L-shaped dormer can extend over that existing structure to create meaningfully more usable floor area than a single rear dormer alone. This option depends on the structure below being suitable to support the additional dormer, and on planning permissions specific to that combination.
Daylight, ventilation and access upgrades, fitted with careful weathering.
A well-positioned rooflight can bring in significantly more daylight than a comparable dormer window, without altering the roofline — but getting the weathering detail right around the opening matters more than with a standard window, since a poorly fitted rooflight is one of the most common sources of loft leaks. We treat the flashing and junction detailing as carefully as the structural opening itself.
Tile, slate, leadwork, membranes and junction details coordinated around the new opening.
Any new opening in a roof — whether for a dormer or a rooflight — has to tie back into the existing tile, slate, or membrane covering cleanly, and this junction detailing is where the majority of post-completion leaks originate if it's rushed. We coordinate this stage specifically around getting these junctions right, not just closing up the opening.
Insulation, airtightness, plastering and neat trims finished as part of the room.
The reveal around a new dormer or rooflight — the internal lining, insulation, and trim — affects both the thermal performance of the room and how finished it looks day to day. Getting insulation and airtightness right at this stage also matters for preventing condensation around the opening later.
Leak diagnosis and correction around existing dormers or poorly fitted roof windows.
A leaking dormer or roof window is often the result of a detailing issue rather than a fundamental design fault, which means it can usually be corrected without rebuilding the whole structure. We diagnose the actual point of failure — flashing, sealant, or a structural movement issue — before recommending a fix, rather than resealing symptomatically.
This depends on where the loft currently lacks headroom or daylight. A dormer adds both floor area and standing height in a specific section of the roof; a rooflight adds daylight and ventilation without altering the volume. We assess which of these — or a combination — actually solves the problem you're describing.
Rear dormers are generally less visible than front or side alterations, which matters both aesthetically and for planning purposes. We consider sightlines from the street and from neighbouring properties as part of the design, particularly where planning conditions restrict visible change to the roofline.
This depends on the size and position of the alteration, whether the property is in a conservation area, and any previous permitted development already used on the property (which can affect what's still allowed). We check this specifically for your property rather than assuming a rear dormer is automatically permitted development.
The point where a new dormer or rooflight meets the existing roof covering is detailed with appropriate flashing, membranes, and lead or equivalent weathering materials specific to your roof type (tile, slate, or flat sections). This detailing is the single biggest factor in whether the alteration stays leak-free long term.
Rooflights in particular can lead to significant summer overheating if specified without consideration for orientation and glazing performance. This is addressed through glazing specification, and in some cases opening/ventilation provision or external shading, considered at the design stage rather than as an afterthought.
This typically covers insulation and airtightness around the new opening, plastered reveals, and trims finished to match the rest of the room — so the alteration reads as a finished part of the space rather than a visible retrofit.
Yes — this is a common combination where a rear dormer provides the main additional space and headroom, while rooflights elsewhere bring in extra daylight to areas the dormer doesn't cover.
Rooflights that don't project significantly above the roof plane typically fall under permitted development, but this can still depend on the property's specific planning history and whether it's in a conservation area. We confirm this during assessment rather than assuming.
A dormer's larger window area can contribute to overheating if not specified with this in mind, similar to rooflights. This is factored into the design — glazing specification and ventilation — rather than being an issue only addressed after the room is built.
The large majority of leaks around dormers and roof windows come from detailing at the junctions — flashing, sealant, or membrane continuity — rather than the window or dormer structure itself failing. This is why remedial diagnosis focuses on those junctions first.