A guest on the second floor of a seaside B&B pushes the sash up to let the heat out, and the gap is wide enough for a toddler to climb through. That gap is exactly what the 100mm rule is there to stop.
Quick answer: Window restrictors limit how far a window opens, usually to no more than 100mm, so a child or vulnerable adult cannot fall out. HSE applies this to care homes, schools and public buildings. For HMOs, councils such as Hammersmith and Fulham ask for it on upper-floor windows with low sills, releasable in a fire.
Where the 100mm figure comes from
HSE's safety bulletin on window restrictors covers organisations that look after people at risk of falling from windows: NHS trusts, care homes, schools and public buildings. It describes restrictors as limiting the opening, usually to not more than 100mm. That is the benchmark everyone works to.
There is no single national statute for hotels and HMOs that says "fit one at 100mm". The duty comes from the Health and Safety at Work Act 1974, your risk assessment and, for HMOs, the licence conditions set by your council. Read the HSE source here: HSE window restrictor safety bulletin.
Which windows actually need one
Councils write their own HMO standards, so check yours. As one worked example, Hammersmith and Fulham asks for restrictors on windows at first floor and above where the sill is less than 1.1m from the floor. See the wording in LBHF HMO guidance on external doors and windows.
| Window | Restrictor? |
|---|---|
| First floor or above, sill under 1.1m | Yes, under council HMO guidance such as LBHF |
| Care home, school or hospital room | Assess every window within reach (HSE) |
| Ground floor guest room | Usually a security question, not a fall one |
| Fire escape window | Only if it releases in a fire |
The fire catch: it has to release
A restrictor that traps someone in a burning room is worse than no restrictor. LBHF asks for devices that an adult can override if no children are present or in a fire. That is why locking cable restrictors, which release with a key or tool, suit rooms with an escape window better than a fixed bolt.
Make sure staff or tenants know where the key lives. A key in a locked office does nothing at 2am.
Where installs go wrong
Three repeat problems. First, the gap is never measured. A product described as a 200mm cable restrictor has a 200mm cable. The gap you get depends on how it is fixed to your window, so measure the finished opening and adjust the fixing until it is about 100mm.
Second, old side-stay restrictors with plastic spacers. HSE flagged potential failure of these in its bulletin, so check any you inherit. Third, nobody looks at them again. HSE does not set an inspection interval, so write your own into the maintenance log, for example a check at each room turnover.
What to buy
Kent Traders stocks Astroflame locking cable window restrictors, tested to EN 13126-5:2011, in white at £9.01 and brown at £10.12 (prices checked 6 October 2026). Shop the full window restrictors collection, or browse safety protection for corner guards and trickle vents. If you are kitting out rooms at the same time, the hotel supplies range keeps it to one order.
Same duty-of-care logic applies to hot water. We covered it in anti-scald care home showers.
FAQs
There is no single national rule. The duty comes from the Health and Safety at Work Act 1974, your risk assessment and your council's HMO licence conditions, which often ask for restrictors on upper-floor windows.
HSE describes restrictors as limiting the opening, usually to not more than 100mm. Measure the actual gap once fitted.
Hotels owe guests a duty of care, and a risk assessment on upper-floor rooms, family rooms and rooms with low sills will usually point to restrictors. Check with your insurer and licensing officer.
It can if it cannot be released. Councils such as LBHF ask for devices an adult can override in a fire, so use locking restrictors with a key or tool release on escape windows.
Not necessarily. HSE covers Great Britain, but HMO licensing is set locally, so check your own council.
HSE does not set an interval. Add a check to your maintenance log, for example at every room turnover or quarterly for common areas.
Fitting restrictors across a hotel or HMO?
Locking cable window restrictors, trade priced and delivered from the UK.
