Put the whole extension in their garden
This is the one that stops people in their tracks. A four metre extension, full height, standing on the lawn, seen from the kitchen door.
Choose a size
Drag to walk round it
Opens your camera. Point it at the lawn and walk back.
Once it appears: one finger drags it, two fingers turn it. It does not need a clear floor. Drop it where it fits and judge the size against what is already there.
How far can you go?
Single storey rear extensions in England have hard numbers. Here they are, against the actual house.
Let them scan it and hold their own phone up
The camera view only works on a phone. Point the code at their camera, and this page opens on their handset. That is the moment the idea sells itself.
Permitted development rights described here are for houses in England and were current at the time of writing. They do not apply to flats or maisonettes, and are restricted or removed in conservation areas, National Parks, Areas of Outstanding Natural Beauty, on listed buildings, and where an Article 4 direction is in place. Always confirm with the local planning authority before work starts. Building Regulations approval is required whether or not planning permission is.