FENSA (Fenestration Self-Assessment Scheme) is a government-authorised competent person scheme for replacement windows and doors in England and Wales. FENSA-registered installers can self-certify that their work complies with Building Regulations (Part L thermal and Part N safety) without needing a separate building control application.

Do I need FENSA for replacement windows?

All replacement windows and external doors must comply with Building Regulations. You either need: a FENSA (or CERTASS) registered installer who self-certifies compliance, OR a building control application with your local authority (at additional cost). Without either, the work is uncertified and will cause problems when selling.

How much does FENSA registration cost?

FENSA annual membership is typically £400–£600 depending on company size, plus a per-installation notification fee of approximately £30–£40 per installation. The total annual cost depends on the volume of jobs. For a typical installer doing 40+ jobs/year, FENSA is significantly cheaper than building control.

What is the alternative to FENSA?

The alternative is a building control application for each installation, typically costing £200–£400 per job. Building control will inspect the windows for thermal compliance (Part L U-values), safety glazing (Part N) and ventilation (Part F trickle vents). This is more expensive for multiple installations but may suit very occasional work.

What happens if windows are installed without FENSA or building control?

Uncertified window replacement is a Building Regulations offence. When selling the property, the solicitor will request FENSA certificates or building control completion certificates. Without them, the seller may need to apply for retrospective building control regularisation (costing £200–£400+) or the buyer may reduce their offer.

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FENSA Registration Calculator — Window & Door Compliance

Compare FENSA registration costs against building control route for window and door replacements. Calculate annual savings and break-even point.

Jobs per year

Avg windows per job

Avg doors per job

How We Calculate This

This calculator compares the annual cost of FENSA registration against building control applications for window/door replacement work.

FENSA cost

Annual membership fee + per-installation notification fee x number of jobs per year.

Building control cost

Per-job building control application fee x number of jobs per year. The break-even point is the number of jobs where both routes cost the same.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.