Each month, PTE's Knowledge Hub team will explore the complexities and technical challenges British project architects face every day.

At a recent design review for a low-rise housing scheme, a colleague posed what should have been a straightforward question: “How high should the window cills be?” You’d think we’d know by now. But the room fell quiet. Not because nobody knows - but because nobody really agrees. It’s a classic case of too many rules, not enough clarity.

It turns out there’s no simple answer. Window design today is a battlefield of overlapping requirements, risk assessments and aesthetic ambition. It’s a space where Approved Documents clash like gladiators: Part K says don’t fall out, Part M wants easy access, Part O says don’t overheat, Part Q wants to keep burglars out, and Part F wants fresh air in.

And then there’s the HSE, whose recent report - snappily titled A Study of the Effectiveness of Guarding to Prevent Falls Through Window Openings - chimes in with a physics-based perspective. Its conclusion? A cill height of 1100mm protects almost everyone from toppling out. But anything lower than 800mm? Practically pointless.

Yet we’re architects. We don’t just follow rules - we design for people. A bedroom window that’s safe but impossible to see out of while seated (or under 10) fails. So, the PTE approach? Fixed glazing below 1100mm where it counts. Let the light in. Let the eyes wander.

As for cleaning them - that’s for another column.