Require advertisements for property to include the floor (and ground) area.

Advertising is already a pretty strict domain by necessity. We used to allow just about every lie under the sun in advertising, and it was universally bad. We systematically broke that down one by one over many decades, and we're still doing it! I mean, these days you can't even sell tobacco without saying "hey we're trying to kill you btw".

This is a very well-trodden path, I think.

While that would be useful it would not change it from a landlords-market - which is what the comment I replied to was about.

…and define rules for the measurement taken, like no stairs, count areas below roof sloping or outside on balconies etc. with a lower factor, exclude shared areas and cellars/attics, and so on.

Not measuring correctly, not publishing the measurements, or publishing incorrect measurements outside an error margin gets counted as deception. Offer a simple reporting contact point to citizens to report landlords or offers that violate this regulation without a lawsuit.