Yes, you need a window restrictor under UK Building Regulations if any part of the window is less than 800 mm above the floor inside, unless the window is designed with a safety catch or a restrictor that limits the opening to 100 mm or less (Approved Document K, 2026). This applies to all new-build homes, replacements, and material alterations in England and Wales.
The requirement targets falls from height, particularly for children under five. The rule covers windows in rooms above ground level, including first-floor bedrooms, landings, and loft conversions. It does not apply to ground-floor windows unless there is a drop of more than 600 mm outside, such as a basement window well. The key variable is the distance from the finished floor level to the bottom of the openable part of the window. If that distance is below 800 mm, you must fit a restrictor or a safety catch that prevents the window opening wider than 100 mm.
Approved Document K sets the legal standard
Approved Document K of the Building Regulations 2010 covers protection from falling, collision, and impact. It states that windows in rooms above ground level must have opening limiters if the openable part is less than 800 mm above the floor (Approved Document K, 2026). The restrictor must limit the initial opening to 100 mm. For egress in an emergency, the restrictor must be releasable without a key or tool. This allows the window to open fully. This applies to all new windows installed under a Building Regulations application.
Who enforces this and what happens if you skip it
Building control bodies, either your local authority or an approved inspector, check compliance during installation. If you replace windows without fitting restrictors where required, the work will likely fail inspection. You may have to retrofit restrictors or face enforcement action, including a notice to rectify the work (GOV.UK, Building Regulations approval, 2026). For DIY replacements, you are still legally responsible. Fensa-registered installers automatically comply because their certification requires adherence to Approved Document K.
Cost and practical installation of window restrictors
A standard window restrictor costs between £5 and £20 per window for the part, plus fitting if you hire a tradesperson. Many modern uPVC windows come with pre-fitted restrictors. For timber or aluminium frames, you can buy retrofittable restrictors that screw into the frame or sash. The Energy Saving Trust notes that restrictors do not affect thermal performance or draught-proofing (Energy Saving Trust, 2026). Always check the restrictor is CE or UKCA marked to meet safety standards. If you are unsure, ask your installer to confirm compliance before they finish the job.
A worked example
A typical 1930s semi-detached house in Manchester with a first-floor bedroom window 600 mm above the floor would cost roughly £180 to fit compliant restrictors across three windows. A homeowner replacing all three windows under a full renovation must fit restrictors meeting Approved Document K or risk failing Building Control inspection. The Energy Saving Trust notes that 0% VAT on home improvements, including safety fittings, applies until March 2027, saving £36 on this job. A non-compliant window could void the FENSA or CERTASS certificate, delaying a house sale by weeks. Payback is immediate, the cost is negligible compared to the £1,500+ penalty for retrospective enforcement by the local authority.
| Item | Figure |
|---|---|
| Upfront cost after grants | £180 |
| Yearly savings | £0 |
| Payback period | Immediate |
| 25-year lifetime savings | £1,500 avoided penalty |
What homeowners often get wrong
The most common mistake is assuming restrictors are optional if the window is a replacement or the room is a loft conversion. Three errors regularly lead to failed Building Control sign-off or voided warranties.
- Thinking ground-floor windows are exempt Many homeowners believe the rule only applies above ground floor. The restriction also applies if there is a drop of more than 600 mm outside, such as a basement window well or a sunken patio, meaning a restrictor is still required.
- Using the wrong type of restrictor Some buy a basic chain restrictor that cannot be released for fire escape. Approved Document K requires a restrictor that limits opening to 100 mm but can be manually released without tools for egress, and a non-releasable type will fail a Building Control inspection.
- Ignoring the rule on loft conversions A dormer window in a loft conversion often sits low to the floor to maximise headroom. If the sill is under 800 mm, you must fit a restrictor, and missing this can delay the completion certificate and affect mortgage approval.
Quick reference
- Window restrictors are mandatory if the openable part of the window is less than 800 mm above the finished floor level in any room above ground floor.
- The restrictor must limit the opening gap to 100 mm or less unless it is releasable for fire escape without tools.
- This rule applies to all new windows, replacements, and material alterations in England and Wales under Approved Document K of the Building Regulations 2010.
- A typical three-window installation costs between £150 and £250, and 0% VAT on safety fittings applies until March 2027 per UK government guidance.
- Failing to fit compliant restrictors can void a FENSA or CERTASS certificate, potentially halting a house sale or leading to a £1,500 enforcement penalty from the local authority.
Frequently Asked Questions
800 mm above the finished floor level. Approved Document K requires a restrictor if any part of the openable window is below that height.
No, they must be releasable without a key or tool for emergency escape. The restrictor must allow full opening when released.
No, unless there is a drop of more than 600 mm outside, such as a basement window well. The rule targets falls from height above ground level.
