A different rule from balconies
Unlike stair, balcony or terrace guardrails — governed by the NF P01-012 standard — whether a window guardrail is needed comes from a separate rule set by French building regulations: it doesn't concern a fall height, but the height of the sill, meaning the solid wall section below the window opening.
The 0.90m threshold
If the combined height of the sill and the frame (the window's fixed casing) reaches 0.90m, no additional protection is required: the sill's own height is enough to act as fall protection. Below that threshold, an added guardrail becomes necessary to secure the opening.
Two questions not to confuse
This 0.90m threshold answers a specific question — is protection needed — which differs from the question of what height the guardrail should be once fitted, if protection turns out to be necessary. The first determines the requirement, the second determines the structure's sizing. Confusing the two is a common source of misunderstanding.
How to measure a low sill at home
To check whether your window is affected, measure the height from the finished floor to the bottom of the glazed section (top of the frame): if that total is under 0.90m, protection should be planned. The most frequently affected configurations are ground-level windows, low bay windows, and some older windows with a reduced sill — a useful check before even requesting a quote.
Once protection is decided: what height?
If protection is required, it follows the same dimensional principles as a balcony guardrail — 1m as a reference — adjusted to the exact opening configuration and the existing sill height. The precise figure is confirmed at the quote stage for your window. See our window guardrail page for installation options.
Frequently asked questions
How do I know if my sill is "low" without complicated maths?
Simply measure the height between the finished floor and the bottom of the window's glazed section. If that figure is under 0.90m, your window falls under the low-sill rule.
Is a window with an 0.85m sill really affected?
Yes, technically: the threshold is set at 0.90m, so any height below that — even by a few centimetres — falls under the case requiring protection.
Does this rule apply to roof windows or skylights?
The low-sill rule concerns standard vertical windows. Roof windows fall under other considerations (accessibility, roof pitch) beyond the scope of this article — check case by case.