Loft Conversion Cost Calculator

Loft Conversion Guide

What's the key thing to understand about Conversion Types and Costs?

Velux (roof light) conversion: £20,000–35,000. Only adds Velux windows, insulation, flooring, and staircase — no structural changes to the roof. Suitable where sufficient head height exists (minimum 2.2m from floor to ridge). Dormer conversion: £35,000–60,000. Adds a box dormer to one or both sides, significantly increasing usable floor space and headroom. Most common UK loft conversion type. Hip-to-gable: £40,000–65,000. Extends a hipped roof to create a gable end — often combined with a dormer

What's the key thing to understand about Planning Permission?

Most loft conversions fall under permitted development (PD) — no planning permission required if: total additional volume does not exceed 40m³ (terraced/semi) or 50m³ (detached). No extension beyond the existing roof slope facing a highway. No balcony or raised platform. No materials that make the exterior look different from the existing house. If your property is in a conservation area, listed building, or if you have used up PD rights previously, you will need full planning permission. Always check with your local planning authority before starting work, since PD rights vary by property type and some local authorities have removed them entirely via an Article 4 direction. A pre-application enquiry (typically £100-300) is a low-cost way to confirm what's allowed before committing to detailed drawings.

What should I know about Structural Requirements?

All loft conversions require Building Regulations approval — this covers structural calculations (floor joists, ridge beam, steel beams), fire safety (30-minute fire doors, mains-wired interlinked smoke alarms), escape windows (minimum 0.33m² openable area), insulation to current standards, and the staircase design. A structural engineer's calculations are required as part of the Building Regs submission. Budget £500–1,500 for the structural engineer and £1,000–2,000 for the Building Regulations application and inspections, which cover roughly six visits from a Building Control officer across the project (foundations/steels, floor joists, insulation, first fix, and completion). Skipping Building Regs approval is a false economy — it complicates any future sale, since solicitors and buyers routinely ask for a completion certificate for loft conversions.

What should I know about Return on Investment?

A well-executed loft conversion typically adds 15–20% to the value of a property — often more than the conversion cost in areas with high property values. In London and the South East, adding a bedroom and en-suite via loft conversion is one of the most cost-effective home improvements. The payback period depends heavily on local property prices: in areas where bedrooms add significant value, ROI is excellent. In lower-value markets, the cost may exceed the added value. Estate agents and surveyors typically estimate a well-executed loft conversion adds 15-20% to a property's value, though this varies enormously by region — London and the South East see the strongest returns, while some Northern and rural markets see closer to 5-10%. Get a local estate agent's opinion before committing to a spec beyond what the local market will actually reward.