French property prices are often quoted in euros per square metre, particularly when comparing prices from one region to another. While this is a useful rough guide, it is not the only measure that should be considered when looking at a property you are thinking of buying.
More important are the disposition and layout of the rooms. Older properties may include a lot of wasted space in the form of corridors, hallways or landings on upper floors, that may be difficult to convert into habitable space. And generally a room that communicates directly with another is inconvenient, except perhaps for a living room/dinnng room or a bedroom/dressing room.
Note however that mezzanines, balconies, loggias and areas where the ceiling height is less than 1.80 metres are not defined as 'habitable space' under the terms of the 'loi Carrez' of May 1997 which introduced new regulations for calculating the surface area of properties sold within a co-ownership complex (principally apartments, holiday villas etc). These often provide additional usable space, for example, as a sleeping area (mezzanine) or additional dining/living space in the case of a (closed-in) loggia or terrace.
When they put their property on the market, vendors are obliged to employ an expert to undertake a series of technical tests (including gas, electricity, lead, asbestos etc) usually referred to as the 'expertises' and which include a certificate of the 'surface loi Carrez' - the official habitable area. The expert's dossier should be made available to intending purchasers before they enter into a commitment to buy.
The expert's measurements should be accurant to within 5% and if it is found to be less then the buyer may have a right to a reduction in the price.
The diffrence between the actual habitable space and the 'surface loi Carrez' may be considerable and both dimensions can be quite legally mentioned in property advertisements. So if a property sounds interesting, it is worth a visit rather than simply relying on the printed description.