Measure the Cargo, Not Just the Glass: The Patio Door Access Audit to Do Before You Order

There is a particular kind of silence that follows the moment a new patio door is fitted and the sofa is still sitting in the front garden under a tarpaulin. It is not a glazing problem. The glass is lovely. The U-value is excellent. The door simply does not have a big enough hole in it for the thing that needs to go through.

Patio door buying guides are very good at telling you about solar gain, frames and security. They are much less good at telling you to measure the cargo. If you are ordering a door in late 2026, before the winter delivery slots tighten up, do the access audit first. It takes half an hour and it can save you a very expensive argument.

Start with the cargo, not the catalogue

Walk around your home and make a list of everything that might ever need to pass through the new opening. The obvious ones are a sofa, a dining table, a mattress, a wardrobe and a piano. The less obvious ones are a mobility scooter, a pram, a wheelchair, a large plant pot, a Christmas tree and the garden furniture you currently drag through the house every April.

For each item, write down three numbers: its longest dimension, its height and its depth. If the legs or arms come off, note that too. A sofa with removable feet is a different beast from one with a solid oak frame. A piano is not a DIY measuring challenge; if that is on the list, speak to a piano mover before you order anything.

Then think forward. The door you buy this year might need to work for a household with a pram, a walking frame or a wheelchair in five years. If step-free access is part of the plan, check the clear opening with an occupational therapist or access specialist, not just with the installer who happens to be on site.

Clear opening width is the number that matters

The catalogue usually quotes the overall frame size or the structural opening. Neither tells you what will actually fit through. What you need is the clear opening width: the narrowest gap between the frame and any obstruction when the door is open in the way you intend to use it.

On a sliding door, the clear opening is not the full width of the unit. It is the width of the moving panel minus the frame and any overlap. On a bifold, the stacked panels eat into the opening, and the stack can sit inside or outside the room. On French doors, one leaf usually opens and the other stays fixed unless you unlock both. Measure the actual gap, not the promise.

Stand at the door with a tape measure and check the width at the narrowest point. Then check the height from the top of the threshold to the underside of the frame. If the door has a raised track or a drainage upstand, measure from the highest point you will have to lift the item over, not from the floor.

The diagonal trick, and why it is only a rough guide

If you are trying to get a boxy object through a rectangular opening, the limiting measurement is sometimes the diagonal rather than the width. Measure the clear width and the clear height of the opening, then work out the diagonal. A quick way is to use a piece of string: hold one end at the bottom corner of the opening and stretch it to the opposite top corner. That length is the doorway diagonal.

If the object's smallest cross-section is bigger than that diagonal, it is not going through. If it is smaller, you have a chance, but it is not a guarantee. Sofa arms catch on frames. Door handles reduce the gap. The thickness of the wall and the track means you cannot always rotate the item in the way you imagined. The diagonal trick rules out hopeless cases; it does not replace a site survey.

The threshold, the track and the handle all count

A patio door is not just a hole with glass in it. The threshold is part of the opening. Some sliders have a low track, some have a deeper one, and some flush thresholds still have a small upstand for weather performance. Measure the height of that upstand because a heavy item has to be lifted over it while someone is holding the other end.

The handle matters too. A chunky handle on the opening leaf can reduce the usable width, especially if the door opens back against a wall or a radiator. Check how far the handle projects and whether the door can open far enough to give you the width you measured. If the door swings inward, make sure the room behind it is clear.

The route from the van to the room

The doorway is only one part of the journey. The sofa has to get from the van, through the gate, along the path, up the steps, through the hall and around the corner. Any of those can be the pinch point.

Measure the narrowest part of the external route: the garden gate, the side passage, the porch and any steps. Note the surface. Gravel, loose paving and a sloped path all make a heavy lift harder. If the only parking is on the street, count the distance and any kerbs. If the patio door is at the back of the house, work out whether the item comes through the house or around the side, and whether the garden steps are wide enough for two people and a sofa.

Inside, check every doorway, hallway turn and stair. A Victorian terrace can have a generous patio opening and a hallway that turns like a corkscrew. A bungalow may be easy at the back door but tight at the front. Measure the route as it is, not as it looks on a floor plan.

A simple pre-order measuring checklist

• Clear opening width at the narrowest point, with the door open as you will use it.
• Clear opening height from the top of the threshold to the frame.
• Threshold upstand and track depth.
• Handle projection and how far the door opens.
• External route: gate, path, steps, gravel, parking distance and kerbs.
• Internal route: every doorway, hallway turn, stair and low ceiling.
• Cargo dimensions: longest side, height, depth and removable parts.
• Future access needs: pram, wheelchair, mobility scooter or walking frame.
• Photographs and a rough sketch with every measurement marked.
• Written confirmation from the supplier of the clear opening dimensions, not just the frame size.

When a wider slider is worth the extra cost

If the access audit shows the clear opening is marginal, decide now rather than after installation. A wider slider, a different sliding configuration or a bifold may give you the clear width you need. Bifolds can open up a wide span, but the stacked panels need somewhere to live, either inside or outside. French doors may be cheaper, but one fixed leaf can leave you with a narrower usable opening than you expected.

The extra cost of a wider door is often less than the cost of discovering the problem later. Retrofitting a wider opening can mean lintels, making good, new flooring and a fresh building control conversation. But do not overspend for the sake of it. If your cargo is a couple of dining chairs and a small table, a standard slider is fine. The point is to make the decision with numbers, not hope.

The moment before you sign

Before you pay a deposit, ask the supplier to put the clear opening dimensions in writing. If the installer says it will be fine, ask them to show you the measurement. If the showroom model looks generous, remember that the frame, track and handles all take a bite out of the opening. The door is not just there to let light in. It is there to let life through, and life sometimes arrives in the shape of a three-seater sofa.