The interesting question is what happens when the battery dies
Two kinds of product share this name. A retrofit lock sits on the inside of your existing door and turns the thumbturn you already have, so the cylinder, the key and the door are unchanged. A replacement lock takes over the cylinder itself and can offer a keypad or fingerprint reader outside. The question that should decide the purchase is neither: it is what happens at zero battery — a key override, an emergency terminal that takes a power bank, or nothing at all. We compare fitting type, cylinder compatibility, credentials offered, and the failure path when flat.
Retrofit · fingerprint, app and Bluetooth · magnetic auto-lock · 6 cells · no drilling
A retrofit lock that mounts over an existing cylinder with no drilling and no cylinder change. Signstek states fingerprint, app and Bluetooth access, magnetic auto-locking, and six batteries; the existing key still works from outside.
Best for: A first smart lock, and particularly a rented flat where nothing may be altered.
≈ 130 €
Matter over Thread · Apple Home Key · rechargeable Li-ion, 6 months stated · no drilling
A retrofit lock with native Matter over Thread, so it joins Apple Home, Google Home and Alexa without a vendor bridge. Aqara states Apple Home Key support, a rechargeable battery quoted at up to six months, and a quiet unlocking mode.
Best for: A household already running Matter or Apple Home, that wants the lock to be part of it rather than another app.
≈ £200 / 170 €
Retrofit · up to 18 unlock methods · full metal body, 50% slimmer · 9 months stated
SwitchBot states a next-generation motor, a full metal housing half as thick as its predecessor, up to eighteen unlocking methods with the Matter hub and touchscreen keypad, and a rechargeable main battery quoted at up to nine months.
Best for: A door where the indoor unit needs to be as unobtrusive as possible, on a household that likes options.
≈ 180 €
Retrofit · built-in Wi-Fi, no bridge · Matter · charged at the door · months per charge
Nuki's fifth-generation lock puts Wi-Fi inside the unit, so remote access needs no bridge or hub, and adds Matter alongside Bluetooth. The battery charges at the door with the supplied cable rather than coming off it.
Best for: A household that wants remote access without adding another box to the hallway.
≈ £189 / 233 €
