A loft conversion is usually the best value room you can add to a house, because it changes the bedroom count without touching a square inch of garden. What it costs depends almost entirely on type, and type depends on your roof.
The prices
Check the head height first
Before anything else, measure from the top of the ceiling joist to the underside of the ridge at the highest point. The rough working number is 2.2 metres.
Above that and most conversion types are open to you. Below it and you are looking at either a mansard, which rebuilds the roof, or lowering the ceilings in the rooms underneath. Both are considerably more expensive, and both change the answer to whether the job is worth doing.
Modern trussed roofs, common on estates built from the 1970s onward, are a different problem. The W shaped trusses fill the loft and cannot simply be cut out, so the roof has to be restructured. It is doable and we do it, but it costs more than a traditional cut roof.
Timescales
- Velux: 4 to 5 weeks
- Dormer: 6 to 8 weeks
- Hip to gable: 7 to 9 weeks
- Mansard: 8 to 12 weeks
- L shape dormer: 10 to 14 weeks
Most of the work happens above your existing ceilings and is reached from scaffold outside, so you can normally stay in the house throughout. The disruptive part is cutting in the staircase, which is a few days near the end.
Fire regulations are not optional
Turning a two storey house into a three storey one triggers fire regulations. In practice that means fire doors on the rooms coming off the stairwell, mains linked smoke alarms and a protected escape route down the stairs.
This is the part a suspiciously cheap quote skips. It is also the part that keeps your family alive in a fire and the part that a surveyor will pick up when you sell. If a quote does not mention it, ask why.
Planning
Velux, dormer, hip to gable and L shape conversions are frequently permitted development, subject to volume limits and where the dormer sits on the roof. Mansards usually need a full application because they change the roof shape so substantially. All of them need building regulations approval regardless.
Is it better value than an extension?
Usually, if what you need is a bedroom. A loft conversion adds a room and often an ensuite for less than a two storey extension costs, and it does not lose you any garden. If what you need is a bigger kitchen and a family space, a loft does nothing for you and a rear extension is the answer.
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