GUIDE

Loft conversion ranges: Velux to mansard

Last checked September 2026 · Compiled UK mid bands, before spec, region, VAT and extras · Published by Rodway Labs

Not a quote. Figures in this guide are illustrative UK cost bands compiled for planning. They are not a tender, valuation or building-regulation ruling. Get written quotes on the same drawing. Project Cost Lab (Rodway Labs) is not a contractor, architect or quantity surveyor. The mid figure is a band, not a price; the VAT toggle is a display choice, not a ruling.

Why form matters more than the word “loft”

Homeowners often search for a single “loft conversion cost”. In practice the envelope, steel, staircase and finishes decide the band. A rooflight conversion that keeps the existing ridge is a different job from a full mansard that rebuilds the roof plane. This lab prices lofts by useful floor area and stores separate compiled mid points for common UK forms.

Use the figures below to size a conversation with an architect or designer, then collect written quotes on the same drawing. Do not treat a mid point as a tender.

Compiled 2026 mid £ per m² (before factors)

Velux / rooflight — about £1,550 / m² (default example 22 m²). Least change to the roof form; headroom and staircase still drive cost.
Rear dormer — about £1,850 / m² (default 28 m²). Common suburban route when you need standing height at the eaves.
Hip-to-gable — about £2,050 / m². Extra masonry and roof work where a hipped end becomes a gable to unlock floor area.
L-shape dormer — about £2,150 / m². Two planes, more junctions, more weathering detail.
Mansard — about £2,400 / m². Heavier structural and roofing package; often the top of the domestic loft ladder in published guides.

These mid points sit before specification, region, access, occupation, listed uplift, VAT display and contingency in the calculator. Change those knobs and the illustrated band moves — sometimes by more than the gap between Velux and dormer.

What usually sits inside the band

Typical published packages assume a weathertight conversion with insulation to current standards, plasterboard finishes, basic electrics and heating extensions, and a straightforward staircase. Exact inclusions vary by quote. In this lab, tick ensuite, a higher-spec staircase, planning, Party Wall or professional fees if those apply to your job — they are allowances, not measured bills of quantities.

What usually sits outside

Full roof rebuilds beyond the loft form, complex steel where trusses were never meant to be cut, asbestos removal, chimney stack issues, main-drainage diversions and temporary accommodation are classic band-breakers. Conservation-area or listed constraints can add design time and specialist materials; the listed slider is a blunt +15% illustration, not a heritage fee quote.

Headroom, stairs and Building Regulations

Useful floor area is not the same as the footprint of the loft void. Stairs eat space on the floor below; fire escape and insulation details affect both cost and programme. This site does not decide whether your loft qualifies under permitted development or needs a full application — that is a planning and Building Control matter for your property.

How to use the calculator for a loft

Pick the form that matches the drawing (Velux through mansard).
Set useful m², not the whole roof void.
Choose spec, region and access honestly — tight terraces and Inner London move the band.
Leave VAT as a display choice until your supplier confirms treatment; the toggle is not a ruling.
Add contingency; loft openings still find surprises.

Open the loft range calculator