A changing room fails twice a day, at the start of the shift and at the end, and almost never in between. That timing points at the number you should actually design around. It is not the total on the induction register, it’s how many people need to use the locker room. Two sites with identical headcounts can need very different changing provision. Especially if one runs staggered starts and the other brings everybody through the gate at once.
Get the number wrong and it shows up fast. Too little bench and floor space and people change standing in the aisle. Or skip the unit completely and travel home in dirty workwear. Too much and you have given up site footprint the programme needed somewhere else.
The two capacity numbers that size a changing block
Most undersized changing rooms come from counting one thing when there were two things to count. Locker capacity and bench capacity are driven by different populations, and they fail in different ways.
- Locker demand tracks everyone working on site during that phase. Personal clothing and effects need somewhere secure to sit all day, not just at shift change
- Bench and circulation demand tracks the largest group that arrives, changes and clears the room inside the same short window
- Drying demand tracks the share of that crew working outside in weather. Which on UK sites is seasonal rather than constant
Separating those three figures is what prevents the familiar compromise: a unit with enough lockers and nowhere to sit. It also explains why a crew that grows by a third can suddenly need a second unit. It’s always better to add a new space than try and fit more locker doors. The locker wall absorbed the growth. The floor did not.
What UK sites are expected to provide for changing and clothing storage
HSE guidance sets the need for a changing area against two triggers: whether workers have to wear special clothing, and whether they can reasonably change somewhere else. Where either applies, the space should let men and women change separately, and it should have seating along with facilities for drying clothing where that is needed.
Storage is treated as a separate outcome. Workers need somewhere their clothing and personal effects stay safe. HSE accepts that on a small job this can be a secure site office, or even a van. But only if the space provided genuinely stays secure. That stops being workable once several trades share a site, partly because an office is not somewhere people can change. Guidance is also clear that contaminated protective equipment is kept apart from everyday clothing.
On duties, contractors provide welfare for the workers under their control and clients have to make sure those arrangements exist. The decisions taken early in project planning rather than after mobilisation. Rules vary by project and location. A site-specific reading of what applies to your work is worth confirming with your principal contractor or the relevant authority.
One practical consequence is easy to miss. The guidance is written as outcomes rather than as a locker count per head, so there is no official ratio table to copy across. Sizing comes back to your own crew and your own arrival pattern every time.
Locker columns are the real capacity lever inside a fixed footprint
A locker container is a fixed box. Changing units for construction sites are commonly built on a footprint of roughly 7.2 square metres, from a 3 m by 2.4 m unit, or 14.4 square metres from a 6 m by 2.4 m unit, with a net interior height of 2.40 m. Those footprints repeat across the industry because they travel and stack predictably.
Inside that box, capacity is set by how the wall is divided rather than by the size of the room. Locker banks running along both long walls can be specified as single, double, triple or quadruple compartments, fitted with rods, shelves and hooks, with benches down the centre so the aisle stays clear while people are seated.
Comparing site welfare cabins on floor area alone hides this completely, because two units of the same size can serve very different crew numbers depending on how the locker wall is subdivided and where the benches sit.
The trade-off tends to be discovered in November. A quarter-height compartment holds a folded change of clothes and a pair of boots. It will not hold a soaked jacket and let it dry. Pushing locker count up by subdividing the same wall reduces the air each garment has around it. So locker density and drying performance pull directly against each other. A site with a lot of external work usually does better with fewer, taller compartments. And a dedicated hanging space than with a dense bank of small doors.
Drying wet workwear needs three things, not one
A heater in the corner is not a drying area. Wet clothing dries when there is heat, air moving across the garments. You need ventilation and a route for the moisture to leave the room. Miss the third and the unit just becomes humid, which is why a drying space with no extraction turns stale within days and why the clothes are still damp the next morning.
That makes a few build details matter more in a drying and changing unit than they would in a dry-trade cabin. An insulated envelope, such as a 50 mm sandwich panel wall, holds the heat you have put in. Instead of losing it overnight which is reall important when it comes to drying. Fixed electrical fittings do the working part: LED lighting, IP44 rated switches suited to a damp room, and an electric extractor sized for the space. Fixed electric heating is also safer than a portable unit here, because draping wet clothing over a heater is a fire risk and a fixed heat source is easier to keep garments clear of.
Drying capacity itself is measured in metres of rail and in hooks with air between them. Not in the wattage of the heater. Two jackets pressed together dry far slower than the same two jackets a hand’s width apart, which is the single most common reason a nominally heated drying area does not work. Worth noting that the heating side of this is seasonal but the air movement is not. A sealed unit with no extraction is uncomfortable in July for entirely different reasons.
Contaminated workwear roughly halves your usable locker count
Keeping contaminated protective equipment away from everyday clothing is straightforward as a principle and awkward as a layout problem. In practice it means the same person needs two storage places rather than one, and it means a route through the unit that does not carry dust from the dirty side to the clean side.
This is where locker counts quietly go wrong. The number was taken from the crew list at one locker per person, without allowing for the trades that need separated storage. Double compartment lockers, or a divided bank with a marked dirty side and clean side, take up no extra floor area but have to be planned in at specification, not retrofitted once the unit is on site.
Where a shower sits on the same block, the sequence works better still, because work clothing can be left on the dirty side and the worker washes before reaching their own clothes. That only helps if the layout actually enforces the order. A shower reached by walking back past the clean lockers does not.
Combined units against a separate changing block
Some changing units carry more than one function. A 14.4 square metre locker container can be laid out with two enclosed toilet cubicles, a shower cabin, a central washbasin and a changing area with two benches and integrated lockers, all served from one water and drainage connection point.
That configuration suits a tight footprint, a crew small enough that one shower does not create a queue, and a set-up where fewer service connections genuinely simplify the site. It suits a large crew much less well. A combined unit puts a wet, humid room next to the space where dry clothes are stored, and the whole block drops out of use when the sanitary side needs servicing. Once the crew is large enough for that to bite, a dry changing and locker unit alongside a separate sanitary block scales more predictably and keeps the wet and dry sides properly apart.
What runs out first as a crew grows
The pattern below is a planning heuristic rather than a standard. The bands overlap, and the real trigger is nearly always the arrival pattern rather than the raw headcount.
| Crew pattern | What usually runs out first | Layout response |
|---|---|---|
| Small gang, arrivals spread through the morning | Nothing, provided storage is secure | Secure lockers within an existing site office can be enough |
| Single crew, one start time | Bench length and floor space | Prioritise seating and aisle width over locker count |
| Large crew, one start time, one entrance | Door and aisle congestion | Split provision across two units rather than one larger room |
| Mixed trades with dust-generating work | Usable locker capacity | Separated dirty and clean storage per affected worker |
| Mostly external work through winter | Drying capacity | Hanging space with heat and extraction, taller compartments |
| Crew size stepping up and down by phase | Fit between unit and crew | Add or remove units by phase instead of one oversized room |
Where the block sits decides whether it gets used
HSE guidance asks you to think about positioning before work starts: whether facilities will have to be moved during the project, access for cleaning and maintenance, and how long it takes someone from the furthest part of the site to reach them, which should be as short as possible. It also suggests placing washing facilities near rest and eating areas so they are actually used.
Changing provision adds one more siting rule of its own. The block belongs on the route between the gate and the work, not off to one side. A changing room that adds a detour gets used at the start of the shift and skipped at the end, and skipping it at the end is precisely how contaminated clothing ends up in vehicles and at home.
Whatever floor space you sized for has a habit of disappearing afterwards. A changing unit sitting near the gate with a clear aisle is an obvious place to leave tools, deliveries and materials, and once that starts the bench capacity you calculated stops existing. Worth deciding at set-up where the overflow storage actually goes, because it will go somewhere.
Common questions
Do we need a changing unit if workers arrive already in work clothes
The need is tied to whether special clothing has to be worn and whether people can reasonably change elsewhere. Where the crew arrives and leaves in their own clothes and nothing on site contaminates them, secure storage may be the real requirement rather than a full changing room. Where PPE and dust are involved, changing on site is what keeps contamination off everyday clothing.
Does drying have to happen in a separate room
HSE guidance asks for an area where clothing can be stored securely and wet items can dry. It describes an outcome, not a dedicated room, so a defined drying zone within a changing unit can satisfy it as long as the wet garments hang with air around them and the moisture has somewhere to go. A separate room becomes the practical answer when the volume of wet workwear is large enough that humidity starts reaching the dry lockers.
How do we provide separate changing on a small mixed crew
The expectation is that men and women can change separately, and on a small crew that is usually met with a partitioned unit or a second compact unit rather than with a timed rota. A rota depends on everybody following it during the two busiest moments of the day, which is exactly when it breaks down.
How do we plan changing capacity when the crew size changes through the project
Size for the phase in front of you rather than the programme peak. Because these units are built as repeatable boxes, adding a second unit for a busy phase is usually more workable than specifying one oversized room that sits half empty for the rest of the job.
Specifying the block
Four answers settle most of the specification: how many people change at the same time, how many need storage across the phase, how much of the crew works outside, and whether dusty trades need separated storage. Those four decide footprint, locker configuration, drying provision and whether sanitary functions belong inside the same box or next to it.
Module-T is a portable cabins supplier and modular construction specialist in the United Kingdom, and for more than 15 years the Module-T has been designing and manufacturing high-quality modular buildings and portable cabins for professional and commercial use, including changing rooms, locker containers and combined sanitary units. Send your crew numbers, phase dates and site constraints through Get A Quote, or contact the UK office on +44 20 4587 1172 or info@module-t.co.uk to work the layout through before the units are ordered.
