In an effort to standardise the rules in Luxembourg used as the basis for calculating habitable floor area, the Luxembourg Institute for Standardisation, Accreditation, Security and Quality of Products and Services (Institut luxembourgeois de la normalisation, de l'accréditation, de la sécurité et qualité des produits et services - ILNAS) has established a standard for determining the floor area of dwellings, whether they are single-family units or located in multi-unit residential building, and regardless of whether they are new or old.

The purpose of this standard, which is limited to residential dwellings is:

• to establish a common language between professionals and individuals;
• to facilitate an objective comparison between several pieces of real property;
• to determine and check the the habitable floor area in a dwelling using simple means available to all.

Definitions

Distance

Shortest straight line connecting 2 points.

Area (surface area)

Two dimensional area generally calculated as the product of 2 distance measurements.

Floor

As a general rule, the floor is the lower horizontal structure of a room which constitutes the elements used to delineate a building or part of a building.

Wall

Generally speaking, a wall is the vertical structure which constitutes the elements used to delineate a building or part of a building.

Room

Accessible part of a building which is completely or partially delineated by separating elements, and the floor and/or ceiling of which form(s) an integral part of the building's construction.

Building

Undivided shelter for a space which is completely or partially delineated by containment structures, which are intended for specific purposes of the occupants.

Unusable attic space and/or mezzanine

An attic and/or mezzanine is declared unusable if there is no floor, if the frame takes up too much space, if the height is too low (less than 2 meters), or if it is difficult to access (trapdoor).

Usable converted attic space and/or mezzanine

Usable attic space and/or mezzanine in conformity with the habitability criteria currently in force.

Areas defined by the standard

Calculation of (surface) area

Area is expressed in square meters (m²) 'intra-muros'.

The "intra-muros" area (surface intra-muros- SIM) consists of all of the internal surface areas which are directly or indirectly available to users. This excludes all building components and fixed partitions.

The intra-muros" area relates to the internal outline of all building components measured above the baseboard. This internal outline must be directly visible, accessible and measurable.

The 'intra-muros' area of a floor or part of a floor of the building is the sum of the surface areas of all possible closed polygons whose sides consist of the visible internal faces of the building components, such as:

• façade walls;
• common walls;
• interior walls and shells;
• fixed partitions and columns.

Habitable floor area

The habitable floor area consists of all the parts of the floor related to the primary uses of a building in conformity with the habitability criteria currently in force.

The habitable floor area includes in particular:

• living areas (living room, bedroom, kitchen, dressing room, etc.);
• bathrooms (bathing areas, toilets, etc.);
• interior passageways (hallways, stairs, etc.).

The habitable floor area does not include:

• common areas;
• parts of a room where the ceiling height is less than 2 metres;
• walls, support elements (for example, a pillar) and partitions;
• doorways and windows;
• shafts at the tops of stairs that are not beneath another set of stairs;
• unusable attic space and/or mezzanines;
• usable but unconverted attic space and/or mezzanines;
• cellars;
• garages;
• patios, loggias, balconies, dryers outside the home;
• unheated porches;
• technical ducts and rooms.

Accessory areas

Accessory areas are parts of the floor that do not form part of the habitable floor area but which contribute to making the dwelling practical and pleasant.

These include:

• garages;
• indoor or outdoor parking spaces;
• patios;
• loggias;
• balconies;
• cellars;
• attics;
• attic space;
• previously excluded porches;
• previously excluded rooms.

Calculating habitable floor area

Calculation for multi-unit residential buildings

The habitable floor area is the sum of the:

• living areas (living room, bedroom, kitchen, dressing room, etc.);
• bathrooms (bathing areas, toilets, etc.);
• internal passageways (hallways, stairs, etc.).

In commonhold buildings which have a 'vertical cadaster', habitable floor area is calculated based on the usable cadastral area, from which the areas with a ceiling height of less than 2 metres and the areas occupied by partitions are deducted.

Calculation for multi-unit residential buildings which do not have a 'vertical cadaster', or for individual dwellings

The habitable floor area is the sum of the:

• living areas (living room, bedroom, kitchen, dressing room, etc.);
• bathrooms (bathing areas, toilets, etc.);
• internal passageways (hallways, stairs, etc.).

Tolerance

Regardless of the measurement used, the tolerance—i.e., the margin of error in calculating the habitable floor area—is no more than 5 % (above or below the exact surface area).