What is the building mass index?

Space in 3D: Definition and Calculation

What exactly is the building mass index?

The building volume index is a ratio. It relates the volume of the building to the area of the plot. It therefore does not define how much ground you are allowed to cover, but rather how much space you are allowed to build "into the air".

The technical definition is:

The building volume index indicates how many cubic meters of above-ground building volume are permitted per square meter of usable land area.

Unlike the utilization factor, which thinks in two dimensions (length times width of the rooms), the building volume factor thinks in three dimensions (length times width times height).

How do you calculate the building mass index?

The formula is simple, but its application requires precision.

Formula:

Building volume index = Above-ground building volume ÷ Allowable land area

In practice, you usually use the formula in reverse to check what is possible:

Maximum building volume = Land area × Building mass index

A calculation example:

Imagine you buy a plot of land in an industrial zone.

• Land area: 1,000 m²
• Building volume index according to zoning plan: 3.5

How much are you allowed to build?

Calculation: 1'000 m² × 3.5 = 3'500 m³ .

You are therefore allowed to construct a building with a volume of 3,500 cubic meters. Whether you use this volume as a flat, enormous hall (large footprint, little height) or as a tall tower (small footprint, great height) is relatively flexible in terms of building volume – as long as you comply with boundary distances and maximum building heights.

What counts towards the volume?

This is where things get tricky, and this is where the building mass index reveals its pitfalls.

above-ground volume counts . That means everything that protrudes from the ground.

• This includes: the volume of the floors, the exterior walls and often also the attic (depending on cantonal regulations).
• This does not include: The basement, provided it is completely underground, as well as open structural elements such as balconies.

Important: The measurement method is largely standardized by the "Intercantonal Agreement on the Harmonization of Building Terminology" (IVHB), but there are cantonal nuances. If you want to determine the building mass index precisely, you need to check how your canton defines "above ground".

Where is the building mass index used?

Why are there two systems (AZ and BMZ)? The building mass index has specific areas of application.

1. Commercial and industrial zones

This is the primary area of application for the building volume index . In a factory, "living space" is irrelevant. A warehouse might need a ceiling height of 6 meters to stack shelves. If one were to calculate using the utilization factor (living space) here, the high warehouse would be at a disadvantage because it consumes a lot of volume but only has one "floor".

The building volume index is fairer here. It allows the contractor to use the volume flexibly. Whether he builds two floors of 3 meters each or one of 6 meters, the building volume index makes no difference. The volume remains the same.

2. Core zones and protection of the townscape

In old village centers or sensitive locations, municipalities often use the building mass index to protect the townscape.

This isn't about use, but about mass. A new building shouldn't appear more imposing than the neighboring historic houses. By setting a maximum building volume index , the municipality prevents the construction of enormous blocks that would disrupt the village's character. The building volume index acts as an aesthetic regulator here.

3. Hotels and special buildings

building mass index is also often used for hotels or public buildings , as these contain foyers, halls or atriums that are difficult to capture using classic living space calculations.

Difference to the utilization factor (AZ)

It is important not to confuse the building volume index with the utilization index.

• Floor area ratio (FAR): This ratio determines the total floor area . It focuses on the amount of floor space one can walk on.
• Building mass index (BMZ): This regulates the volume of the building structure. It focuses on the building envelope.

An example to illustrate:

You are building a house with a 100 m² floor area and extremely high ceilings (4 meters).

• For the floor area ratio , only the 100 m² of floor space counts. The ceiling height is irrelevant (as long as it remains a single story).
• The building's mass index is entirely determined by its height. 100 m² x 4 m = 400 m³ volume.

The building mass index thus "penalizes" inefficient room heights, while the AZ ignores them.

Strategies: Getting the most out of the building mass index

If you want to build on a plot of land with a fixed building volume index , you can optimize.

• Building underground: Since the building volume index usually only limits the above-ground volume, you can relocate storage areas, garages, or utility rooms to the basement. This saves valuable above-ground volume for the actual use.
• Roof shapes: Depending on the canton, the roof space is factored into the building volume index differently . Sometimes a clever roof pitch can help create volume that is not fully counted (observe knee wall regulations).
• Utilizing the terrain: If the property is on a slope, the definition of "above ground" is often a matter of interpretation. By cleverly shaping the terrain, it is sometimes possible to partially "bury" floors to reduce the building mass index .

Conclusion

The building volume index is the currency of cubic meters. You'll encounter it primarily where commercial use or the preservation of the townscape is concerned. It's more flexible than the rigid floor area ratio, as it allows for freedom in ceiling height – a dream for anyone planning lofts, studios, or warehouses.

However, anyone buying a plot of land with a building volume index needs to rethink their approach. You're not buying square meters of living space; you're buying a virtual volume package. Whether you fill this package efficiently or squander it through wasteful architecture is up to you. Before buying, always check the zoning plan to see what the building volume index is and—more importantly—how the municipality defines "above-ground volume."

If you are unsure about the calculation or want to know how many cubic meters are actually feasible on your desired plot of land, it is worth using Loft 's data analysis to recognize the full potential of the plot.

Glossary

• Building volume index (BMZ): A key figure in building law that indicates how many cubic meters of above-ground building volume may be erected per square meter of land area.
• Building volume: The total volume of a building, measured by its external dimensions (length x width x height), which forms the basis for the building volume index .
• Utilization factor (UFF): The counterpart to the building volume factor ; it regulates the permissible floor area (2D), while the BMZ regulates the volume (3D).
• Eligible land area: The area of the property that serves as the basis for multiplication by the building volume index (often minus road clearance areas ).
• Above-ground volume: The part of the building that protrudes from the natural terrain and is relevant for the building mass index .