To ensure consistency, the Real Estate Transactions Act has rules that real estate companies must follow when displaying property floor sizes on advertisements and in contract documents.
Why does it matter?
Some tax breaks have size restrictions, and these sizes will be based on different methods of measurement. The home loan tax reduction, which allows a borrower an annual tax deduction of 200,000 ~ 400,000 Yen for up to 10 years, requires a minimum inside-wall measurement of 50 sqm. Some banks may also refuse to lend on properties with an inside-wall measurement of less than 50 sqm. The acquisition tax payable upon purchase of real estate is slightly lower for residences with a taxable floor area of between 50 to 240 sqm.
 Center-wall measurement (kabe-shin menseki): This is the floor area measured from the center line or middle of the apartment’s outer walls. It does not include balcony space, elevator hallways or car parks, although it may sometimes include a trunkroom storage closet located outside of the apartment. This is the official measurement to be used in all real estate materials including advertisements and contract documents. Support pillars and ducting space may also be included.
Because this measurement includes part of the solid wall, it may be slightly larger than the actual usable space within the apartment. As a very, very rough guide this size may be around 5% larger than the inside-wall measurement.
 Inside-wall measurement (uchi-nori menseki): This is the floor area measured from the inside walls of the apartment. Although it is closest to the actual usable space of the apartment, this measurement is used only for official registration purposes. This size is recorded on the property title documents, and will also be indicated in contract documents.
 Taxable floor area (kazei menseki): This is used by the tax office to calculate property taxes. It includes a share of the building’s common areas, such as lobbies and elevator halls, so the size will be bigger than the actual apartment size.
 Center-wall measurement (kabe-shin menseki): As above, this is the house’s total floor area measured from the center-line of the exterior walls. This measurement is recorded on the property title documents as well as the building approval and completion certificates. This is the official measurement to be used in all real estate materials including advertisements and contract documents. If the building’s eaves or balconies are over 2 meters deep, the portions over that size may also count towards the floor size.
 Taxable floor area (kazei menseki): As above, this is for tax purposes only. The size is usually the same as size recorded on the property title documents, although there are cases where the tax office uses a different measurement.
 Footprint / building area (kenchiku menseki): This is the size of land that the building takes up. It is used when calculating footprint ratios (kenpeiritsu) that apply to land. If the 2nd floor is larger than the first floor, eg. with overhanging rooms, the 2nd floor size will be used for the footprint.
 Registered size: The size registered on the property title. In many cases this is not going to be the same as the actual size, especially for land that was registered several decades ago when measurements where less precise than they are today.
 Surveyed size: The land size as measured by a surveyor. This is an accurate measure, although it costs money to obtain a survey so many land sellers will not bother to do this. Usually a buyer will obtain a proper land survey as part of the building process.