Wohnflächen-Checker - Contrôle de la surface habitable

Living area sounds simple, but it is not. The square-metre figure in your lease is by no means always the space you actually live in. Under the German Living Space Ordinance a balcony or terrace usually counts at only 25 percent, a room under a sloped ceiling only partly, and an unheated cellar or garage not at all. That is exactly where the checker comes in: it calculates your real countable area, room by room.

You add your rooms and pick a type for each: regular room, room with a sloped ceiling, balcony, terrace, loggia, heated or unheated conservatory, cellar, garage or other. For each type the tool knows the correct WoFlV factor and the relevant legal basis - a balcony under Section 4 No. 4 at 25 percent, a regular room under Section 2 at full area.

Sloped ceilings are the classic case where landlords like to miscalculate. Areas under a ceiling height of one metre do not count at all, between one and two metres only half, and only from two metres up fully. The checker asks for exactly these three height ranges and works out the countable area for you, instead of you crouching in the attic with a folding ruler and calculator.

The real highlight is the deviation calculator. Enter the area stated in your lease, your base rent, the service charge prepayment and the start of the tenancy, and the tool compares it with the actual area. If the flat is more than 10 percent smaller than agreed, the rent is reduced according to the Federal Court of Justice, usually based on the total rent. The tool shows the deviation, the monthly and yearly reduction and the refund since the start of the tenancy, minus payments that are already time-barred.

Important context: this is a calculation aid, not legal advice and not a court ruling. The 10 percent threshold comes from case law, but every case is different, and actually enforcing a reduction is something you should clarify with a tenants association or a lawyer. The checker hands you the numbers you need to even start that conversation.

The calculation runs in your browser, with no data transfer. Your inputs stay in this browser under your account until you sign out, and you can print the breakdown. You can load presets for typical flats to get a feel for the maths, and adjust your own rooms until the area is right. In the end you have a transparent breakdown that lets you meet your landlord as an equal.