Building Regulations for replacement windows: Part L, F and K

Part L: thermal performance

Replacement windows in England must achieve a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B or better. Our standard A-rated 28mm double glazed specification (BFRC licence 3220) clears both routes, so any window you build in the designer complies thermally by default. Triple glazing simply exceeds it.

Part F: ventilation

Since June 2022, new windows must provide background ventilation no worse than the windows they replace, and where the old windows had no vents, habitable-room replacements are generally expected to add them. The decision it drives: fit trickle vents unless you know the room is ventilated another way. Our trickle vents guide covers equivalent areas and the 2000EA/4000EA options.

Part K: safety glazing

Glazing in critical locations must be safety glass: anywhere within 800mm of floor level, and within 300mm of a door edge up to 1500mm high. The decision: check every low cill and door-adjacent pane, and specify toughened or laminated units there. Doors themselves are always critical, which is why we only glaze doors with safety glass.

Part B: escape windows

In a house, Approved Document B asks for an emergency escape window (or an external door) from habitable rooms, not kitchens, on the ground floor and, in a house with one stair, on floors up to 4.5m above ground level. A ground floor room that opens straight onto a hall leading to an exit doesn't need one, and nor does an upstairs room with direct access to a protected stairway. Floors more than 4.5m up are covered by other rules, so check with building control.

An escape window needs an unobstructed openable area of at least 0.33m², at least 450mm high and 450mm wide, with the bottom of the openable area no more than 1100mm above the floor. It should stay open without being held. When you replace a window that is big enough to escape through, and would be an escape window in a new house, the new one should give at least the same potential for escape, or, if the old one was bigger than it needed to be, it can be reduced to those minimums.

Part K is a separate rule: where an opening window is above a drop (upstairs, for example) and its cill is less than 800mm above the floor, Approved Document K asks for guarding up to 800mm, so people can't fall out. If you're not sure what a room needs, ask your fitter or building control before you order.

The designer's fire-escape option sizes the side-hung opener for the escape opening. At least one side opener must be at least 650mm wide and 920mm high in its own section, so a top vent or transom above it can't shrink the opening; on a window with two side openers one is enough, though the designer makes both that size, as they normally match. It makes the window as big as that needs, and won't let a window too small for it be ordered as a fire escape. How high the window sits in the wall is set by the opening and your fitter, not the designer.

Every layout with a side opener offers the fire escape option, for example the Full Side Opener, Side Opener + Fixed, Classic Combination and Triple: Side / Fixed / Side. Tick it in the designer and it checks the opener size for you. Every side opener has egress hinges as standard.

Who checks all this?

Compliance is certified either by an installer registered with a competent-person scheme (FENSA, Certass and similar) who self-certifies the work, or by a building-control application when you fit yourself or use an unregistered fitter. The FENSA and self-certification guide walks through both routes for supply-only buyers.

If someone else is fitting your windows, our hiring a fitter guide covers what to ask them and what to send them, and our window fitting guide shows how a uPVC window goes in.

Frequently asked questions

Do Building Regulations apply if I'm just replacing like-for-like?

Yes, replacing a window (frame and all) is controlled work even when nothing else changes. Repairs like swapping a sealed unit within an existing frame are not.

Will your windows pass Building Regulations?

Thermally, yes by default: the standard spec exceeds Part L's requirement. Every side opener has egress hinges as standard. Ventilation, safety glazing and an opener big enough for escape depend on choices you make per window (vents, toughened glass, the fire escape option), and the designer offers each one where it's relevant.

What happens if replacement windows were never signed off?

It typically surfaces when you sell the house: buyers' solicitors ask for FENSA certificates or building-control completion. Retrospective regularisation or indemnity insurance are the usual fixes, but it's cheaper to certify properly at the time.

Do the same rules apply in Scotland and Wales?

Wales runs its own versions of the Approved Documents with similar but not identical figures; Scotland uses the Building Standards system with different processes. The guidance above describes England. Check the national rules if you're elsewhere.

Sources

The official documents and schemes this page refers to. Rules change, so check the current version, and ask your fitter or building control if you're not sure.

• Approved Document B: Fire safety (volume 1 covers dwellings): escape windows
• Approved Document F: Ventilation: background ventilation and trickle vents
• Approved Document K: Protection from falling, collision and impact: safety glass and guarding
• Approved Document L: Conservation of fuel and power: U-values and energy ratings
• FENSA: competent person scheme for windows and doors
• Certass: competent person scheme for windows and doors