Dormer or rooflight loft conversion: which is right for your house
The choice between a rooflight conversion and a dormer is really a question about your ridge height, and you can answer most of it yourself with a tape measure and half an hour in the loft.
A rooflight-only conversion is cheaper, faster and usually needs no planning permission, but only works where the existing ridge height already gives you standing headroom. A dormer costs more and uses permitted development volume, but creates genuine usable floor area on roofs that are otherwise too tight.
Start with the measurement that decides it
Measure from the top of the existing ceiling joists to the underside of the ridge board. You will lose roughly 150–250mm to the new floor structure and finish, and 250–350mm to rafter-line insulation and plasterboard at the ceiling. So a 2.6m raw measurement becomes something like 2.1m finished at the ridge, which is workable. Under about 2.4m raw, a rooflight-only conversion produces a room where you can only stand comfortably along the centre line, and that's when a dormer starts earning its cost.
What a rooflight conversion gives you
Insulation at the rafter line, a new floor, a staircase, fire-rated separation and two or three rooflights in the rear slope. No external structure, no scaffolding-heavy roof surgery, no change to the roof shape, and — outside conservation areas — no planning application. It's typically the quickest conversion on site and much the least disruptive externally. Light quality from rooflights is genuinely excellent, better per square metre of glass than a vertical window, because you're capturing sky rather than the fence opposite.
What a dormer gives you
Vertical walls and full-height space across the dormer's footprint. On a standard rear-facing box dormer that turns a cramped roof space into a proper double bedroom with room for wardrobes and a usable en-suite. It also permits standard vertical windows, which read better internally and make the space feel like a room rather than an attic. The trade-off is cost, external appearance and, in most cases, using up your permitted development volume.
Cost comparison
As a broad guide for this part of south-west London, a rooflight conversion of a straightforward cut roof is commonly in the £30,000–£45,000 range depending on finish, stair position and bathroom. A rear box dormer conversion on the same house typically runs £45,000–£70,000. A hip-to-gable plus rear dormer, common on 1930s semis around Hampton and Whitton, sits higher again. Converting a trussed roof adds meaningfully to either figure because of the structural work.
Planning implications of each
Rooflights on a rear or side slope of a house outside a conservation area are generally permitted development if they project no more than 150mm and don't sit above the ridge. Dormers need to fit the 40m³/50m³ volume allowance, sit below the ridge and be set back from the eaves. Inside a conservation area — much of Richmond borough — a dormer requires full planning permission and even rooflight positions may be controlled. That difference alone decides the question for some properties.
Appearance and resale
A well-designed rooflight conversion is invisible from the street and adds a bedroom without altering the house. A large box dormer is very visible from the rear and, done cheaply, looks it — flat-fronted, over-wide and clad in the wrong material. Done well, with the cheeks in matching tile or lead and the proportions set to the windows below, it's unobtrusive. Estate agents locally will tell you buyers care much more about whether the room is genuinely usable than about which method created it.
The roofing risks in each
Rooflight conversions are low-risk from a weathering point of view: a trimmed opening and a factory flashing kit, correctly installed, is a reliable detail. Dormers concentrate risk — flat roof falls, cheek cladding, the apron at the base, the soakers up each side and the junction at the top. When we're called to a leaking loft conversion, it's the dormer base or the cheek junction nine times out of ten, and it's almost always a detailing shortcut rather than a material failure.
Getting the roof assessed first
Before committing either way, it's worth having the roof itself looked at. If the covering is 70 years old with fatigued nails, the sensible sequence is to re-roof and convert together rather than insulate the underside of a roof that needs stripping in three years. We survey across Hampton, Hampton Hill, Twickenham, Teddington, Whitton and the wider TW and KT postcodes and will tell you plainly what condition the covering is in.
FAQs
Whichever produces a properly usable room. On a tight roof, rooflights alone may create a space buyers read as a study rather than a bedroom, while on a high-ridge house they're indistinguishable in value terms.
Technically yes, but you'll pay twice for scaffold, insulation and finishes. If the headroom is marginal it's cheaper to decide once at the outset.
Modern units with the correct flashing kit for your tile profile are reliable. Leaks almost always trace to the wrong flashing kit, insufficient tile clearance below the unit, or an unsupported trimmed opening.
A rooflight conversion is often six to eight weeks; a dormer conversion typically eight to twelve, depending on structure and finish.
Related reading
More on comparisons and the jobs that usually come with it.
- Full re-roof vs overlaying an existing coveringComparisons
- Repair or replace? How to decide honestlyComparisons
- Roofer or general builder: who should do the workComparisons
- Pitched roof repairs: what's worth fixing and what isn'tRepairs & problems
Looking for something specific? See our roofing services, cost guides, common roof problems or the areas we cover.
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