Dormer vs Rooflight Loft Conversions: Which One, and What It Means Structurally
A rooflight conversion is the simplest and cheapest. A dormer adds real headroom and space. Here is how the main loft conversion types compare, and the structural work each one involves in a London home.
In short: a rooflight (Velux) conversion keeps your existing roof exactly as it is and sets windows flush into the slope, so it is the cheapest and least disruptive, but it only works if you already have enough head-height in the loft. A dormer builds a box out from the roof to create standing space and more floor area where the roof would otherwise be too low, at a higher cost. Whichever you choose, a loft conversion is a structural project: the old ceiling was never designed to be a floor, so new beams are almost always needed. Tell us what you are planning and a structural engineer will explain what your roof can take.
The four common loft conversion types
Most loft conversions in London fall into one of four types. They differ in how much space they add, how much they cost, and how likely they are to need full planning permission rather than falling under permitted development. The diagram below shows the roof profile of each.
Rooflight (Velux) conversion
The simplest and cheapest. The existing roof is left as it is, and windows are set flush into the slope. There is no change to the roof shape, so it is the least likely to need planning permission and the least disruptive to build. The catch is head-height: a rooflight conversion only gives you a comfortable room if the loft already has enough height under the ridge. If it does not, you are looking at a dormer or a more extensive option.
Dormer conversion
A dormer builds a structure outward from the roof slope, most often on the rear, to create a section of full-height wall and a flat or pitched roof above. This adds both headroom and usable floor area, which is why rear dormers are the most common conversion on London terraces. A dormer is a bigger structural job than a rooflight: the projection has to be framed, supported and weatherproofed, and it changes how loads travel down through the roof.
Hip-to-gable conversion
Where a roof has a sloping "hipped" end, that end can be built up vertically into a flat gable wall, squaring off the roof space and reclaiming the volume the slope used to take. It is common on semi-detached and end-of-terrace homes and is often combined with a rear dormer. Because it changes the roofline, it more often needs a planning application.
Mansard conversion
A mansard rebuilds the roof into a steep, almost vertical slope with a shallow top, giving the largest gain in internal space of any option. It is a substantial construction and design project, and in London it is frequently the route taken in conservation areas because a well-detailed mansard can suit period streets. It almost always requires full planning permission and careful structural design.
Dormer vs rooflight: how to choose
The decision usually comes down to three things:
- Head-height. If you already have generous height under the ridge, a rooflight conversion may give you the room you need for far less money. If you are short of standing height across much of the floor, a dormer earns its cost by creating it.
- Space and layout. A dormer adds floor area you can actually stand and put furniture in, which can be the difference between an occasional room and a proper bedroom with an en-suite. A rooflight keeps the sloping ceilings.
- Planning and appearance. A rooflight is visually the most discreet and the easiest to get approved. A dormer, hip-to-gable or mansard changes the roof shape, so it carries more planning risk and, on the front slope or in a conservation area, usually needs consent.
Cost follows structural effort, broadly rooflight, then dormer, then hip-to-gable and mansard, but these are general patterns rather than fixed rules, and every roof is different.
The structural side of a loft conversion
This is the part homeowners underestimate. A loft is not a floor waiting to be furnished; it is a roof space. Turning it into a habitable room changes how the building carries load, which is why a loft conversion needs a structural engineer whichever type you choose. The main structural elements are usually:
- A new floor. The existing ceiling joists were sized to hold up a ceiling, not to be walked on. New, deeper floor joists are added, commonly carried on new steel beams built in at floor level so the new floor does not overload the walls below.
- Roof support. Opening up the roof for a dormer, or removing internal timbers to make usable space, means the roof loads have to be picked up elsewhere, often on a new ridge beam and supporting steels.
- Dormer and opening framing. A dormer or a new stair opening needs beams and trimmers designed around it, with the load paths carried safely down to the existing structure and foundations.
- Checks below. The new loads travel down through the walls and into the foundations, so the engineer checks that the structure beneath can take them.
These members are sized in a set of structural calculations for Building Control. If your project also involves opening up rooms lower down, our guide to removing a load-bearing wall covers the same principles at a smaller scale.
Planning permission and permitted development
Two approvals sit behind most loft conversions, and they are separate. Planning permission is about whether you are allowed to build it and how it looks. Many rooflight and modest rear dormer conversions can fall under permitted development, but that is not a given, especially in London. Permitted development rights are often removed on flats and maisonettes, in conservation areas, and where an Article 4 direction applies, all of which are common across the capital, and there are volume limits and design conditions. Front-facing dormers, hip-to-gable and mansard conversions more often need a full application. Always confirm the position with your local planning authority before assuming permitted development.
Building Regulations
Building Regulations are the second approval, and they apply even where planning permission is not needed. A loft conversion has to satisfy requirements for structural adequacy, fire safety and means of escape (converting a two-storey house to three storeys changes the escape and stair requirements), floor loading, insulation and, in practice, sensible headroom over the stairs and floor. Our structural calculations and details form part of that Building Control submission, and we respond to any queries during approval.
London terraces and party walls
Most London loft conversions are on terraced or semi-detached homes, which means the work often affects a shared party wall, cutting in beams, or building a dormer up to the boundary. Where it does, the Party Wall etc. Act 1996 usually applies and you will need to serve notice on the neighbours and, in many cases, appoint a party wall surveyor. It is a separate legal process from planning and Building Regulations, but it is worth knowing about early because it affects your programme.
What to do next
If you are weighing up a dormer against a rooflight, the honest first question is how much head-height your loft already has, because that often makes the decision for you. From there, the structural design is what turns a concept into something Building Control will pass and a builder can actually construct. EMA Structures provides the structural engineering for loft conversions across London, and as a consultancy that both designs and delivers works, we can take it through to construction where you want a single team. Working with an architect on layout and planning? We slot in alongside them. Send us your plans or a few photos of the loft and we will tell you what is realistic.
This article is general guidance, not planning, legal or engineering advice for a specific property. Permitted development rights, conservation-area rules and party wall obligations vary by location and change over time, and every roof and home is different. Always confirm planning with your local authority and have the structure designed for your specific project. For advice on yours, send us your plans or photos and a structural engineer will talk you through it.
Loft conversion questions
Which is cheaper, a dormer or a rooflight loft conversion?
A rooflight conversion, sometimes called a Velux or roof-light conversion, is usually the cheapest option because it keeps the existing roof shape and simply adds windows into the slope. A dormer costs more because it involves building a projecting structure out from the roof, but it adds usable head-height and floor space that a rooflight cannot. The right choice depends on how much headroom your existing roof already has, and what your budget and planning position allow.
Do I need planning permission for a loft conversion in London?
Many rooflight and modest rear dormer conversions can fall under permitted development, but this is not guaranteed. Permitted development rights are removed or restricted on many flats, maisonettes and houses in conservation areas or under an Article 4 direction, which are common across London, and volume limits and design rules apply. Hip-to-gable and mansard conversions, and anything to the front roof slope, more often need a full planning application. Always check with your local planning authority before assuming permitted development.
Do I need a structural engineer for a loft conversion?
Yes. A loft conversion changes how the roof and the floor below carry load, so it needs structural design. The existing ceiling joists were not built to be walked on as a floor, so new floor beams and joists are usually required, often supported on steel beams built into the structure. A structural engineer designs these members and the supports for any dormer or new opening, and provides the calculations Building Control need.
Can EMA Structures design and build a loft conversion?
EMA Structures provides the structural engineering for loft conversions across London, the floor and roof design, steel beam sizing, and the calculations for Building Control. As a consultancy that both designs and delivers works, we can also manage and carry out the construction where you want a single team from design through to completion. We work alongside your architect or designer on the planning and layout side.
Planning a loft conversion?
Send us your plans or a few photos of the loft, and we'll advise whether a rooflight or dormer suits your roof, and what the structure needs, straight answer, no upselling.