What does the Loft Conversion Cost Calculator do?

Estimate loft conversion costs by type (Velux, dormer, hip-to-gable, mansard). See ROI and value added. All calculations are performed in your browser using official UK rates and thresholds.

Does this include waste allowance?

Yes. Most building material calculations include a standard 10% waste allowance. You can adjust this figure depending on the complexity of your project and cutting requirements.

Material prices are indicative estimates based on average UK prices. Actual costs vary by region, supplier, brand and current market conditions. Always get quotes from local suppliers.

Loft Conversion Cost Calculator

Estimate loft conversion costs by type (Velux, dormer, hip-to-gable, mansard). See ROI and value added.

Last updated: April 2026 · Source: GOV.UK — Permitted Development rights for householders

Conversion Type

Current Property Value

Est. Cost

£45,000.00

£35,000.00–£55,000.00

Value Added

£70,000.00

+20% of property

ROI

56%

New Value

£420,000.00

Usually falls under Permitted Development. Building Regulations approval always needed. Party Wall Act may apply for semi/terrace.

Disclaimer

This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.

How It Works

Loft conversion costs depend on the type of conversion and the existing roof structure. The main types are: Velux/roof-light (cheapest, £20,000–£35,000, adds roof windows without changing the roof shape), dormer (£35,000–£60,000, extends the roof vertically to create more headroom), hip-to-gable (£40,000–£65,000, converts a hipped roof end to a vertical gable wall) and mansard (£50,000–£75,000, replaces the roof slope with a near-vertical wall and flat top).

Building Regulations approval is required for all loft conversions, covering structure, fire safety (protected staircase, fire doors, smoke alarms), insulation, means of escape and staircase design. Planning permission is needed if you exceed Permitted Development limits (40 m³ for terraced houses, 50 m³ for detached/semi-detached).

Enter your roof type, desired conversion type and size. The calculator estimates the build cost, additional costs (staircase, en-suite, electrics, plastering) and professional fees. It also checks whether your plans are likely to fall within Permitted Development limits.

Example: Rear dormer on semi-detached, 25 m² floor area

Dormer build cost: £45,000 (mid-range)
Staircase: £3,000–£5,000
En-suite bathroom: £4,000–£7,000
Professional fees and Building Regs: £3,500–£5,000
Total estimated cost: £55,000–£62,000

Source: GOV.UK — Permitted Development rights for householders

Frequently Asked Questions

What does the Loft Conversion Cost Calculator do?: Estimate loft conversion costs by type (Velux, dormer, hip-to-gable, mansard). See ROI and value added. All calculations are performed in your browser using official UK rates and thresholds.
Does this include waste allowance?: Yes. Most building material calculations include a standard 10% waste allowance. You can adjust this figure depending on the complexity of your project and cutting requirements.
Are prices accurate?: Material prices are indicative estimates based on average UK prices. Actual costs vary by region, supplier, brand and current market conditions. Always get quotes from local suppliers.