Part O: Protection From Falling, and The Curious Case of the Easily Accessible Windows

Paul Bainbridge, Technical Director

Paul’s Greenprints   

When it comes to designing, building, or even simply living in a new build home, you’d think the concept of windows would be the most transparent thing you could think about. As our blog explores, however, when it comes to Part O of the Building Regulations, all is not as clear as you’d expect!

You’d be forgiven for thinking that what you’re about to read is a previously unearthed Sherlock Holmes adventure, but sadly, it’s not quite that entertaining.

But, as with everything we share here at The FES Group, we hope our latest blog certainly proves to be just as interesting for you to read, given that when it comes to anything related to the Part O building regulations for Overheating, there are many, many cases where our clients are being tripped up, caught out, and generally left out in the cold.

Part O was initially tagged on with the new Part L back in 2021, which is probably why it still isn’t really as clear as it could be in terms of its stipulations. It’s a new concept to the design and build of residential properties, and to put it lightly in our humble opinion – it needs work.

Of course, Part O gives us a good understanding of Overheating in terms of its causes and how we can mitigate them, but when it comes to the design of buildings in the first instance, there are things that catch out those hard-working designers. They even seem to catch out assessors, so you can understand why we’re so keen to write about this to help you as our existing and potential clients.

The biggest issue catching designers out is the fact that the regulations in Part O (Overheating) actively clash with the regulations in Part K (Falling, Collision, and Impact). That’s reassuring, isn’t it?

Let us explain.

Part O states that the minimum sill height for your upstairs windows (anything above the ground floor) needs to be a minimum of 1100mm from the floor. All clear so far. However, Part K states that the height for the sills must not exceed 1100mm. Much like our attitude in trying to get our heads around this minimum/maximum concept, there’s little tolerance with the height of the windowsills, it would seem.

This ‘set of parameters’ puts designers in a very precise position, so if nothing else, we hope this blog is of use when drawing things up for your upstairs windows! Frustrations and jokes aside, however, it’s worth noting that NHBC will accept +0/-100mm build tolerances on-site, and will not accept guarding solutions (we’re coming to those) where the guarding height is annotated on the design as less than 1100mm, should this info help on-site operatives navigating the design.

With Part O being relatively new, and with housebuilders being keen not to flout the guidance of the more established and well-known Part K, designers will almost always aim to hit below the 1100mm sill height, and, as a result, will often place their upstairs windows of their properties in too low a position to be compliant when it comes to the Part O safety aspect of falling from those windows. You can feel the headache coming on, can’t you?

In our experience here at The FES Group, you’ve got two solutions to this problem. The first solution is to redesign the buildings so that all upstairs windowsills hit that 1100mm stipulation bang on the money – no more, no less. Therefore, everyone who writes the regulations is happy, and absolutely nobody falls to their death from a window. Job done. The second solution is to introduce guarding bars to those windows, in the hope they’ll do the very job you’d expect them to do, given the name.

What doesn’t sit brilliantly with us here at The FES Group, is that if you take a look at supplier websites where guarding bars are sold, whilst you’ll see clear shots depicting an installation of the bar modelled at a 1100mm regulatory height fitting, what those pictures also reveal is that the fitting in question now presents the problem of a gap between the bar and the low sill height that could lead to entrapment. According to regulations (Part O and Part K, because thankfully they agree), there should be no more than a 100mm gap between guarding bars and windowsills to prevent this entrapment – for example, kids, dogs, or drunken relatives getting their head stuck between them. But in all seriousness, whilst the point of the guarding bar is to take us to that holy grail of the 1100mm distance from the floor, it seems that when looking at this from the point of view of the homeowner, the decorative display on supplier websites may actually be a little misleading. This is worth being aware of, we feel.

The great thing for our clients who wish to take this second solution and avoid a redesign, is that guard bars are cheap and undoubtedly solve a problem, but you can understand that because they really aren’t very pretty, homeowners really don’t tend to love them. It’s always worth bearing in mind that all-important end customer, isn’t it? It’s also worth remembering that the apparently cheap ‘per unit’ cost of guarding bars soon racks up when you consider that all windows above ground level would require their placement if the window design wasn’t compliant to that Part O 1100mm minimum.

Of course, these sites are advertising to homeowners who don’t know the regulations, but as our wonderful clients, you absolutely do need to know them. Manufacturers and suppliers should really be more aware and responsible, of course, but perhaps that’s a subject for a future blog…

In some cases, a solution that might be landed on is to factor the use of window restrictors into the building design, because as you’d expect, if a window can’t open far enough, then it would be impossible to fall out of it anyway. But it’s a grey area if this logic can actively fit with regulations, because our perhaps cynical (though entirely practical) take is that as window restrictors always need to have an over-ride function on them so that the homeowner can manually use them for fire escape purposes, this pours figurative water on the claim that they adequately serve as protection from falling. Our advice would be that if you’re going to propose the use of window restrictors in your designs, discuss it first with Building Control, who will happily give you a final say on the matter.

Now, onto easily accessible windows! These sound great if you’re planning on using one to re-enact a scene from your favourite romantic comedy or action film, and they sound even better if you’re considering carving out a career in breaking and entering. Let’s go on that angle, shall we, and see what we can advise to reduce the risk of easily accessible windows posing a security threat whilst navigating that curious Part O document.

All windows located within 3.5m of the ground – so essentially all ground floor windows – are classed as ‘easily accessible’. But let’s not forget the caveats. Any windows within 2m of a flat roof? Or within 2m of a pitched roof of less than 30 degrees? They fall into the classification of ‘easily accessible’ too, and this poses a problem if your designs include such elements as beautiful bay windows, or kerb-appeal additions such as a mono-pitched porch over the front door. Essentially, if you seek the input of many Building Control teams, they’ll say that if there’s a ‘platform’ that someone can climb up onto and get balanced (god forbid someone trying to break into your home should fall and hurt themselves), you’ve got yourself in a situation where you need to consider the risk of easily accessible windows in your design.

We’re sharing this regulatory piece of info with you in this blog because in our experience here at The FES Group, it’s really catching people out. What might look beautiful from a design perspective, can pose a threat to security thanks to the ease with which someone can access the windows above. Our advice in this matter – and to help you keep these elements in your design if and where possible – is to base your argument around the fact that those roofs are not load bearing, and therefore, should not take the weight of anyone who might be using them to access the upper levels of a property. No solid footing – no ability to be used as a platform. In such cases where the manufacturer makes this claim, the result from an attempted break-in is that a criminal is more likely to hurt themselves than access the property. Our hearts bleed for them, they really do.

Again, this is a discussion to have with Building Control and indeed the manufactures of the roof, who will either back you or they won’t with regards to them being a security risk, leaving you either free to keep the design, or giving you the opportunity you to rethink it completely in order to stay compliant.

So, now that we know where we stand (hopefully on a load-bearing base) on what constitutes an easily accessible window, where do we go from here to mitigate risks whilst ensuring overheating isn’t a concern?

Simple – in theory, naturally. It’s stated in the regulations that occupiers of the property don’t leave those easily accessible windows open either whilst the room is unoccupied (or the home as a whole, for that matter), or at night, when of course there’s going to be a higher risk of opportunistic crime. Now, not being able to open your window at night in the height of summer is a little bit of an issue when we consider that Part O is all about overheating, isn’t it? Admittedly, thanks to the micro-climates of the UK, this is less of an issue for developments being built in say Leeds and anywhere upwards. The exact same building that passes an overheating assessment in Newcastle would likely fail in Norwich, so location is always a factor in relation to design. We know… ‘Welcome to the North… be sure to bring a coat’.

Compliant so far? No remedial measures needed. You’re all good.

Not so compliant or likely to fail the overheating assessment? You’re looking at the need to factor in purge ventilation into your designs – an individual fan in the affected room that can be worked by the homeowner as and when overheating occurs, thus negating the need to open the window. Purge ventilation fans are noisy and don’t help with things like odours or the desire for a blast of fresh air, however, so homeowners don’t exactly all rush to use them in place of throwing open the window. But from a regulation point of view, you really do need to be seen as providing that ventilation at least as an option.

Of course, if you want to go extreme with this, you can factor in whole-house ventilation via a Mechanical Ventilation Heat Recovery system. It’s complex, but admittedly not as complex as a Cooling System (comfort cooling or full-on air-con). Those systems are ideal (and necessary) if you’re designing fancy builds in the South, so glass-fronted residences on the banks of The Thames, for example, but again, it’s less likely to be a need in a Mock Tudor mansion in Middlesbrough. Comfort cooling and air-conditioning systems will of course affect the SAP and associated assessments and costs, as well, so it pays to be aware of this in addition to the concept of overheating in relation to easily accessible windows.

Quick point of note whilst chatting with our compliance team… when exploring potential options around the need for windows to remain closed due to being easily accessible, another viable solution is the introduction of wall vents/louvres to achieve additional ventilation requirements by passive means. After all, Part O/CIBSE TM59 encourages any overheating strategy to be as passive as is as practical, so this option certainly keeps the regulators happy. It’s definitely a more feasible option for our friends in the North, at the very least.

Our thoughts to sum up? If you don’t want to be adding the cost, labour, and homeowner anger-generation of making amendments in the post-construction phrase, or you don’t want to be spending half your life on hold to Building Control to find out what you can and can’t get away with, play it safe and get yourself back to the drawing board – literally. Redesign is the way forward, and the team at The FES Group are here to support you with whatever you need as you go about that task.