What is the building mass number?
Die Construction mass number (BMZ) is a central figure in construction planning and describes the Ratio of aboveground building volume to land area. It determines how many cubic meters of construction volume may be built per square meter of land area. This key figure is part of the building application and enables authorities to check the planned building density of a project and ensure that legal requirements are met. In Switzerland, the building mass rate replaces the utilization rate in many new planning and building laws (PBG).
Statutory regulations on building mass numbers
The building mass rate is defined in the Cantonal Planning and Construction Acts (PBG) or in the Planning and Construction Act Ordinance (PBG). Traditionally, it was used to regulate the density of buildings in commercial and industrial areas. However, it is now also relevant in residential construction to promote densification in urban areas. Particularly as a result of the 2014 spatial planning revision, the building mass figure is being used to efficiently use vacant spaces and make better use of existing areas. The exact regulations vary from canton to canton and are adapted to regional conditions.
Calculation of the building mass number
The building mass figure is calculated by dividing the building volume by the eligible plot area. For example, if the construction volume of a building is 300 cubic meters and the land area is 600 square meters large, the result is a building mass figure of 0.5. This formula helps architects and planners adapt the construction volume of a project to legal requirements and ensure that the building density is not exceeded.
What is the construction volume?
To calculate the building mass number, the aboveground construction volumes used, which refers to the space that is above the relevant terrain. To determine the building volume, the floor area of the building body (i.e. length times width) is multiplied by the height of the building.
Eligible parts of buildings include:
- The basic cube of the building in accordance with its external dimensions
- roof structures (e.g. air conditioners or dormers)
- Canopies and roof projections (with an extension of more than 1.5 m)
- Open components such as balconies
Non-eligible components:
- Covered house entrances
- Open roof incisions or components that are not watertight
- Balustrades, staircases, antennas and photovoltaic systems
- Public transport routes on their own land
- Landfills and green roofs
The precise definition of the eligible building parts ensures that the construction volume is calculated correctly and that only the relevant components are included in the building mass figure.
What is considered decisive terrain?
To determine the construction mass figure, the volume above the decisive terrain taken into account, this corresponds to the volume over the natural course of the property. If the original course has been changed as a result of previous earthworks, the natural terrain of the surrounding area is used. In special cases, such as for planning or development reasons, the relevant terrain can be defined in the building permit process in order to ensure an exact building density.
Difference between building mass number (BMZ) and utilization rate (AZ)
In Switzerland, there are two key figures for regulating Building density of a plot of land: the building mass number (BMZ) and the utilization rate (AZ).
- Construction mass figure (BMZ): It describes the ratio of building volume to land area and thus regulates the maximum permitted building volume per plot of land.
- Utilization rate (AZ): The AZ, on the other hand, determines the ratio of the usable floor area to the land area and thus regulates the maximum permitted floor area on a plot of land.
These two indicators are used to regulate building density and provide building planners and real estate developers with a precise framework for how land can be used optimally and efficiently.
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