Everybody approaches a smart lock as a software problem. Which app, which assistant, does it do auto-unlock, will it nag me about a subscription. All reasonable questions, all completely premature, because before a smart lock is a software problem it is a hardware problem, and the hardware does not care how good the app is.

This lesson arrives, for most of us, at the aluminium sliding door.

Measure this before you fall in love with anything

A normal hinged door is a fairly forgiving host. A sliding aluminium door is a hostile one, and it’ll reject a beautiful lock for reasons that have nothing to do with smarts: the stile width (the metal frame is often too narrow for the lock body), the door thickness, the backset and mortise not matching what the lock expects, and the plain fact that you may need to cut into an aluminium frame to fit anything at all, which is a very different Saturday than "swap a deadbolt."

So the order of operations is the opposite of what the marketing trains you to do. Don’t fall in love with a lock and then hope it fits. Measure the door first, then go looking for a lock that fits it. Stile width, thickness, backset, the existing hardware cutout. Photograph all of it. Bring the numbers, not the vibe.

Once you’ve cleared the fit hurdle, the actual smart options open up — PIN pads, Wi-Fi or Bluetooth, Home Assistant integration, the works — and they’re genuinely good. But every one of them is irrelevant if the body won’t physically marry your frame. I’ve watched people buy twice: once for the lock they wanted, and again for the lock that actually fit, after the first one spent a fortnight in its box judging them.

The boring discipline that saves you here is the same one that saves you everywhere in this hobby. Confirm the constraint before you buy the solution. Measure the door. The app is the easy part and always will be.

THE LEDGER

Cost: one lock — ideally not two

Actual cost: a tape measure and the humility to use it before the checkout button

Survives a firmware update? The app might wobble; the hole you cut in the frame is forever

Verdict: It’s a carpentry problem wearing a software costume. Measure first, swipe second.