The conversation usually starts with a builder or a loft company. They’ve been up the ladder, had a look around, and said a double dormer loft conversion will give you two good bedrooms and a shower room. Next door had one done. It sounds sensible, and it may well be. The question I’d want answered first is whether it’s the right shape for your house, or simply the one that gets offered most.
What is a double dormer loft conversion?
The term is used loosely. Sometimes it means two separate dormers on the back slope of the roof. Sometimes it means dormers on two different slopes, usually the rear and one side. In both cases each dormer turns part of a sloping roof into an upright wall with a flat roof over it, which gives you standing height across more of the floor. On the right house it’s a very good way to get usable rooms out of a roof.
The Guildford townhouse loft conversion.
The Guildford townhouse above is a good example of a roof doing more work without the house growing outwards: a new top floor with a bedroom suite, an office and storage.
What decides whether a dormer works?
Three things decide most loft conversions, and a double dormer is no different.
Head height first. As a rule of thumb you want around 2.2 metres from the top of the ceiling joists to the underside of the ridge, before the new floor and insulation take some of it back. Then the roof structure: a traditional cut roof, built from individual rafters, is usually easier to open up than a modern trussed roof braced across the whole loft. And then the staircase. It has to arrive on the floor below without stealing a bedroom, with enough headroom over it to meet building regulations. If the stair is designed after the rooms, there’s often nowhere sensible left for it.
Then comes the planning position, which is where a double dormer most often comes unstuck.
Do dormers need planning permission?
Many loft conversions are permitted development, which means they can go ahead without a planning application as long as they stay inside fixed rules. For a loft, the main limits are:
- extra volume of no more than 40 cubic metres on a terraced house and 50 on a semi-detached or detached one
- nothing on a roof slope that faces the road at the front
- nothing higher than the highest part of the existing roof
- dormers set back from the eaves, normally by about 20 centimetres where that’s practical
- materials that look similar to the existing roof
- side-facing windows obscure-glazed, and fixed shut unless the opening part is more than 1.7 metres above the floor
- no balconies or raised platforms
Two generous dormers can use up that volume quickly, especially where the roof has been enlarged before, because the allowance counts everything added to the original roof, not just the new work.
A dormer on a side slope that faces a road, as on a corner plot, isn’t permitted development. Neither is any roof extension in a conservation area or the Surrey Hills National Landscape. Those need planning permission, and the council will look hard at how big the dormers look from the street. Two large flat-roofed boxes can make a house look top-heavy, and that’s a fair reason for a council to resist them. Heritage and conservation covers what changes in those areas. If an application is needed, a decision usually takes 8 to 12 weeks after validation.
Even when you’re confident it’s permitted, a lawful development certificate, the council’s written confirmation, is sensible. Building regulations apply whatever the planning route, and in a semi or a terrace, steels bearing on the shared wall usually mean party wall notices. Planning permission vs building regulations explains how the two sets of approvals fit together.
Hip-to-gable vs dormer: which is better?
On a semi-detached or detached house with a hipped roof, a hip-to-gable conversion with a single rear dormer often gives similar space in a calmer shape. The sloping side of the roof is rebuilt as an upright gable wall, which adds volume exactly where the new stair usually wants to go.
Where the loft is already a good size, a single rear dormer might give one proper bedroom and a shower room, which may be all you need. And where the head height is already there, rooflights alone, as in the photograph at the top, leave the roof shape as it is and add the least space and the least upheaval.
I wouldn’t choose between these on price per room. I’d choose on what the rooms need to do, what the stair allows, and what the council is likely to accept on your street. The truth about loft conversions in Surrey goes through how to tell whether your loft is suitable in the first place.
How much does a double dormer loft conversion cost?
As a Surrey benchmark, a loft conversion costs £80,000–£140,000 before VAT, depending on specification. Site conditions and a contingency of 10–15% move it too. Where a particular loft lands in that range depends on how much new structure it needs, whether there’s a bathroom, and the finish.
A shower room in the roof brings plumbing and drainage runs with it, and the position of the soil pipe often decides where the bathroom can go. For the fittings alone, guide prices for a bathroom or en-suite run from £4,500–£9,000 for a standard specification to £10,000–£20,000 for a premium one, VAT included.
Double dormers in Surrey
The houses that get offered a double dormer most often are the interwar semis and the 1960s and 1970s estate houses that make up so much of Woking, Guildford and the towns between them. Their roofs are usually hipped, their lofts are often tall enough, and the stair is nearly always the problem. In the older streets of Guildford, Farnham and Godalming, and anywhere in the Surrey Hills, a conservation area changes the answer, because roof extensions there need a planning application and the council will judge how the dormers read from the road.
If you are weighing one up in a particular town, our pages on loft conversions in Woking and loft conversions in Guildford cover what each council looks for, and the borough planning guides go through the local rules one council at a time.
So is a double dormer the right answer?
The useful question isn’t whether a double dormer is good in general. It’s whether it’s right for this roof, this stair and this street, and whether it has been compared properly with a hip-to-gable or a single dormer before anyone draws it up.
That comparison is quick to make before drawings and slow to make after. It’s part of what the Clarity Pack tests on a loft: the structure, the head height, the stair and the planning position, before the design is fixed. More on the work itself is on the loft conversion page.
If you’d like a second opinion on what you’ve been offered, the free 15-minute Clarity Call is a good place to start.
Questions people ask about double dormer loft conversions
Do I need planning permission for a double dormer? Often not, if both dormers sit on the rear slope, the added volume stays under 40 cubic metres on a terrace or 50 on a semi or detached house, and nothing rises above the existing ridge. A dormer on a slope facing a road, a house in a conservation area or the Surrey Hills, or a roof that has been enlarged before, all take it outside permitted development and into a planning application.
How much does a double dormer loft conversion cost in Surrey? As a Surrey benchmark, a loft conversion costs £80,000–£140,000 before VAT, depending on specification. Two dormers and a shower room sit towards the upper half of that range, and a 10–15% contingency belongs on top.
Is a double dormer better than a hip-to-gable? Neither is better in general. A hip-to-gable with one rear dormer usually gives a calmer roof and puts the extra volume where the stair wants to go. Two dormers suit a roof that already has gable ends, or a house where the side slope faces a neighbour and cannot change. The right answer comes from the stair, the head height and the street, not from a price per room.
