Loft conversion staircase
Roof space turned into a room: the opening framed out, a new staircase built and fitted, and a glass balustrade installed to the landing edge.
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Loft Conversions in Wiltshire
Loft conversions are the cheapest way to add a proper room to a house, because the roof, the walls and the foundations are already paid for. What follows is what the different types cost, when you need planning permission, and what Building Regulations will actually ask of you.
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Before anything else
Is your loft tall enough?
This is the question that decides everything, and it takes two minutes to answer. Measure from the top of the existing ceiling joist to the underside of the ridge.
You need about 2.4 metres to be comfortable. You will lose roughly 200–250mm to the new floor structure, which has to be independent of the old ceiling joists, and another 100–150mm at the ceiling to insulation and plasterboard. Below 2.2 metres a standard conversion gets difficult; below 2 metres you are looking at lowering the ceilings in the rooms beneath or raising the roof, and the cost stops making sense. If that is where you have landed, a garden office in the garden will usually give you the same room for a good deal less, and without living in a building site for four months.
The second constraint is the staircase, and it is the one people forget. Building Regulations want 2 metres of headroom over a stair, reduced to 1.8 metres at the centre of the flight for a loft conversion. There has to be somewhere for that stair to land, and a landing long enough at the top. More loft schemes die on the staircase than on the budget.
What we do: the carpentry. New floor structure, dormer framing, studwork, the staircase and the second fix. You will also need a structural engineer, a builder, a roofer, an electrician, a plumber and Building Control. We are not a design-and-build contractor and will not pretend to be one.
Types
Types of loft conversion
Four types cover almost every house in Wiltshire. Which one you can have is largely decided by your roof, not by preference.
Rooflight conversion
The cheapest and least disruptive. The roof shape does not change at all — you add rooflights, a new floor, a staircase, insulation and finishes. Only works if you already have the headroom, but where you do it is far and away the best value. Almost always permitted development.
Dormer conversion
A box built out from the rear roof slope, giving full headroom and a vertical wall for a window. The standard answer for a Victorian or interwar terrace or semi, and the most common conversion in the country. Usually permitted development at the rear; almost never at the front.
Hip-to-gable
For a house with a hipped roof — one that slopes on three or four sides rather than ending in a vertical gable. The sloping end is built up vertically to make a gable, which creates a great deal of usable space. Usually paired with a rear dormer. Common on 1930s semis and postwar estates.
Mansard
The most expensive and the most transformative: the whole rear roof is rebuilt at around 72 degrees with a near-flat top, effectively adding a storey. Nearly always needs full planning permission. Common in conservation areas precisely because the profile suits period terraces.
Cost
What a loft conversion costs in Wiltshire
Whole-project figures across every trade, not our carpentry alone. Most pages in this market will not give you a number at all.
| Type | Typical range |
|---|---|
| Rooflight conversion, existing usable headroom | £25,000 – £40,000 |
| Rear dormer, one bedroom | £45,000 – £65,000 |
| Hip-to-gable with rear dormer | £55,000 – £80,000 |
| Mansard | £70,000 upwards |
| En-suite, added to any of the above | £6,000 – £12,000 |
What moves it most: whether steels are needed to carry the new floor, how far the soil stack and water supply have to travel for an en-suite, the number and type of windows, and the finish. What moves it less than people expect: the floor area. Adding two square metres to a dormer costs very little once the scaffolding is up.
Planning
Planning permission and permitted development
Do you need planning permission?
Usually not. Most loft conversions are permitted development, which means no application is needed provided the work stays inside the limits: a volume allowance of 40 cubic metres on a terraced house or 50 on a semi-detached or detached one, nothing extending beyond the plane of the existing roof slope on the principal elevation, and nothing higher than the existing ridge.
Side-facing windows must be obscure-glazed and non-opening below 1.7 metres, and materials should be similar in appearance to the existing house.
When you definitely do need it
Conservation areas, listed buildings, and any property whose permitted development rights have been removed by an Article 4 direction — which applies to parts of several Wiltshire towns. Flats and maisonettes have no permitted development rights at all. A mansard almost always needs an application because it changes the roof profile.
Even where you do not need permission, a Lawful Development Certificate is worth the fee. It is the council confirming in writing that the work was permitted development, and it is the document a buyer's solicitor will ask for. Without one, a conversion can hold up a sale years later.
The Party Wall Act
If you are cutting into or building off a shared wall — which a terraced or semi-detached loft conversion almost always does — you must serve notice on your neighbour under the Party Wall etc. Act 1996, normally two months before work starts. It is separate from planning and it is not optional. Serving it late is one of the most common delays there is.
Building Regulations
What Building Control will look at
Permitted development is about what the outside looks like. Building Regulations are about whether it is safe and warm, and they apply to every loft conversion without exception.
Floor structure
Existing ceiling joists are sized to hold up a ceiling, not to carry a room. A new floor goes in, usually on deeper joists spanning between load-bearing walls or off steel beams, and independent of the old ceiling. This is normally where the structural engineer's calculations go.
Part K — stairs
Maximum 220mm rise, minimum 220mm going, pitch no steeper than 42 degrees, every step identical. Headroom 2 metres, relaxed to 1.8m at the centre of the flight for a loft. Handrail 900–1000mm, and no gap in a balustrade that passes a 100mm sphere.
Part L — insulation
A converted roof needs to reach roughly 0.15–0.18 W/m²K. In a rafter-level build-up that means PIR between the rafters with a further layer over or under them, plus a 50mm ventilation gap above if the roof is not a warm-roof build-up. Skimping here is how you get a loft room that is unusable in July.
Part B — fire escape
The one that surprises people. Going from two storeys to three triggers a protected escape route: fire doors on every room opening onto the stair, 30-minute fire resistance to the floor, and mains-wired interlinked smoke alarms. An escape window in the loft room itself if the route is not protected.
Ventilation and condensation
Warm moist air rises, and a loft is the coldest surface in the house. Extract ventilation to any en-suite, a vapour control layer on the warm side of the insulation, and cross-ventilation in cold-roof build-ups.
Sign-off
Building Control inspect at stages — structure, insulation, fire, completion — and issue a completion certificate. Keep it. Like the Lawful Development Certificate, it is what a buyer's solicitor asks for.
Our part
The carpentry, from floor to handover
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Floor and structure
New joists to the engineer's spec, off steels or load-bearing walls, decked and made safe so the rest of the trades have something to work from.
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Dormer and studwork
Dormer framing, cheeks and roof, internal partitions, and the openings formed for windows and doors.
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Staircase
Set out to Part K, made to the opening and fitted. Usually the hardest single element and the one that decides whether the layout works.
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Second fix
Fire doors hung and closers fitted, skirting and architrave, balustrade, and any fitted storage in the eaves — which is the space people always forget to plan for and then wish they had.
Recent work
Loft carpentry we have done
Structural beam and ceiling work
Ceiling opened up, new timber let in and the whole thing boarded and finished ready for decoration.
Questions
Loft conversions — common questions
Do I need planning permission for a loft conversion?
Usually not. Most loft conversions fall under permitted development, with a volume allowance of 40 cubic metres on a terrace and 50 on a semi or detached house, provided nothing extends beyond the existing roof plane on the front elevation and nothing goes above the ridge. Conservation areas, listed buildings, flats and houses whose permitted development rights have been removed always need an application.
Is my loft tall enough?
Measure from the top of the ceiling joist to the underside of the ridge. Under 2.2 metres and a standard conversion becomes difficult; you will lose roughly 200–250mm to the new floor build-up and another 100mm or more to insulation and plasterboard at the ceiling, and Part K wants 2 metres of headroom over the stairs, with 1.8m accepted on a loft stair.
What does a loft conversion cost?
In Wiltshire, a rooflight conversion of an existing usable space typically runs £25,000–£40,000; a rear dormer £45,000–£65,000; a hip-to-gable with dormer £55,000–£80,000; a mansard £70,000 and up. Those are whole-project figures across all trades as of August 2026, not our carpentry alone.
Do you do the whole conversion or just the carpentry?
The carpentry: new floor structure, dormer framing, studwork, the staircase, and the second fix. You will also need a structural engineer, a builder, a roofer, an electrician and a plumber, plus Building Control. We are happy to work alongside a builder you have found, or to say what order things need to happen in.
Talk to us about your loft
Measure joist to ridge, send us the number and a photograph, and Alex will tell you honestly whether it is worth doing.