Estimate the cost of converting your loft with Velux, dormer, hip-to-gable, or mansard options
A loft conversion is one of the most popular and cost-effective ways to add living space to a UK home. Rather than building an extension or moving to a larger property, converting unused roof space into a bedroom, office, or playroom can add 10-20% to your property value while avoiding the disruption and expense of an outward extension. There are four main types of loft conversion in the UK, each with different cost profiles, planning requirements, and structural implications. A Velux (roof light) conversion is the simplest and cheapest, adding windows without changing the roof structure. A dormer extends the roof outward, creating more headroom and usable floor space. A hip-to-gable conversion squares off the sloped side of a hipped roof to maximise internal volume. A mansard conversion raises the party wall and creates a nearly flat roof with a steep back slope, offering the most space but at the highest cost. This calculator uses current UK trade rates per square metre for each type, with optional en-suite bathroom costing.
To estimate your loft conversion cost: 1. Enter the loft floor area in square metres. This is the footprint of the space you plan to convert, not the total roof area. Most loft conversions range from 15 to 40 m2. If you are unsure, measure the length and width of the top-floor ceiling below the loft. 2. Select the conversion type. Velux is cheapest and does not alter the roofline. Dormer is the most popular choice for semi-detached and terraced houses. Hip-to-gable is ideal for houses with hipped roofs. Mansard is the most expensive but creates the most usable space. 3. Toggle the en-suite bathroom option. Adding a shower room or full en-suite to a loft conversion is a popular choice that increases both comfort and resale value, but adds 3,000-8,000 to the project cost depending on specification. 4. Review the budget, average, and premium estimates. The budget figure assumes basic specification with a competitive builder. The average reflects a mid-range fit-out with a reputable contractor. The premium includes high-quality finishes and specialist joinery.
The calculator uses UK trade average rates per square metre for each conversion type: Velux/roof light: 650-1,100 per m2 Dormer: 1,000-1,800 per m2 Hip-to-gable: 1,200-2,200 per m2 Mansard: 1,500-2,800 per m2 Total cost = floor area (m2) x rate per m2 If an en-suite bathroom is included, a fixed cost is added: En-suite: 3,000 (budget) to 8,000 (premium) The cost per m2 output shows the average rate including any en-suite costs, which helps when comparing quotes. These rates include design, structural engineering, Building Regulations fees, labour, materials, and VAT. They are based on RICS Building Cost Information Service data and published Checkatrade averages.
Inputs: Floor area: 20 m2, Conversion type: Dormer, En-suite: No
Inputs: Floor area: 25 m2, Conversion type: Velux, En-suite: Yes
Inputs: Floor area: 30 m2, Conversion type: Mansard, En-suite: Yes
All prices include VAT at 20% and are based on 2026 UK trade averages. London and the South East are typically 20-30% above national averages due to higher labour costs. A loft conversion requires Building Regulations approval regardless of whether planning permission is needed. You will need a structural engineer to design the floor joists, steelwork, and staircase. Party Wall Act notices may be required if you share a wall with a neighbour. Most loft conversions can be completed while you continue living in the property, though expect some noise and disruption. Always obtain at least three itemised quotes from contractors with proven loft conversion experience and check references from previous clients.