Average loft conversion costs in the UK
In the UK, loft conversions generally range from £25,000 for a simple Velux rooflight conversion up to £65,000+ for a complex mansard or hip-to-gable dormer conversion with an en-suite bathroom.
The primary cost drivers are the type of conversion, head height, the requirement for structural steel RSJ beams to support the new floor joists, and building a compliant staircase that meets building control fire regulations.
Types of loft conversions and their structural requirements
Velux / Rooflight conversions are the most economical because the existing roof structure is retained without altering the roofline. Dormer conversions add vertical walls and flat roofs, maximizing usable head height and floor space.
Hip-to-gable conversions replace the sloping "hip" end of a semi-detached or detached roof with a vertical gable wall, ideal for creating a spacious bedroom and bathroom. Mansard conversions remodel the entire roof profile, often requiring full planning permission.
Worked example: 25m² rear dormer conversion with en-suite in the South East
A semi-detached house in the South East converting 25m² of attic space into a master bedroom with a rear dormer, en-suite bathroom (£5,000), and standard staircase.
Core dormer construction at regional rates (£1,750/m²) totals £43,750. Adding the en-suite bathroom brings construction to £48,750. Professional fees (architect, structural engineer, and building control inspection at 12%) add £5,850, bringing total project investment to approximately £54,600.
Inputs for the loft conversion cost calculator
Loft conversion style
Velux rooflight, dormer, hip-to-gable, or mansard.
Floor area (m²)
Total usable internal floor area created in the new loft space.
En-suite bathroom
Select whether to install a shower, toilet, and sink in the new loft.
Finish specification
Standard, good, or premium luxury interior finishes.
UK Region
Regional location applying local trade and builder pricing.
CALCULATOR QUESTIONS
UK loft conversion cost calculator questions
What is the minimum head height for a loft conversion in the UK?
You generally need a minimum of 2.2 metres of clear height from the existing ceiling joists to the ridge beam for a conversion to be feasible without expensive roof raising.
Do I need planning permission for a dormer loft conversion?
Most rear dormers fall under Permitted Development rights (up to 40 cubic metres for terraced houses or 50 cubic metres for semi-detached/detached), provided they do not extend beyond the front roofline.
Do loft conversions require building regulations approval?
Yes. All loft conversions require Building Regulations approval covering structural integrity, floor joist strength, fire escape routes, and insulation (Part L).
How much value does a loft conversion add to a UK house?
A well-designed loft bedroom with an en-suite bathroom can increase the market value of a UK home by 15% to 20% according to Nationwide building society studies.
Do I need a Party Wall Agreement for a loft conversion?
If your home is terraced or semi-detached, you must serve Party Wall notices on your neighbours because structural steel beams rest directly on the shared party wall.