Significant changes have been made to the Regional Stormwater Ordinance (GSVH). These changes relate to when infiltration facilities are required and how their size is determined.

We list the most important changes for you:

Infiltration facility mandatory in construction projects:

Infiltration facilities are always required for new construction, reconstruction or expansion (except when the cadastral plots belonging to one property are together smaller than 120m², infiltration is not required).

Difference between old (2013) and new (2023) regulation :

2013: < 250m²

2023: <120m²

Determine the size of the infiltration facility:

The required size of the infiltration facility depends on the drainage area of the site.

Infiltration volume and area:

The infiltration volume should be equal to 33l/m² of the draining area.
The infiltration surface must be 8% of the total draining surface.

• 2013: 25l/m² and 4%
• 2023: 33l/m² and 8%

Drainage area calculation:

The total drainage area is determined by the sum of several elements:

1. Surface paving: New/reconstruction/expansion (except permeable pavements with a slope <2%).
2. Roof area

! Important exception:

If a storm drain and/or green roof is present, they can be subtracted from the drainage area:

• Deduction Rainwater Tank: If a new rainwater tank is constructed that meets the requirements or if there is already an existing rainwater tank that is connected in compliance, the drainage area can be reduced by 30 m² per connected residential unit. Without residential units, a reduction of 30 m² can also be applied. With larger stormwater sumps and motivated use of stormwater, the draining area can be further reduced.

2013: Deduction of 60m² if compliant HWP is present.

2023: Deduction of 30m² per connected residential unit if compliant HWP is present.

• Green Roof Deduction: With green roofs, connection to a rainwater cistern is not mandatory. If you do connect to a rainwater cistern and the green roof has a buffering capacity of at least 50 l/m², the area of the green roof may be halved when calculating the drainage area for the infiltration facility.

2013: To determine the infiltration surface, the roof surfaces of green roofs are divided by two.

2023: To determine the infiltration area, the roof areas of green roofs that have a buffering capacity of 50 l/m² are divided by two.

The construction of the infiltration facility:

2013: /

2023: According to the new GSVH 2023, the infiltration facility must be constructed above ground unless the permit applicant provides substantiated evidence that underground construction is unavoidable.

Up to a depth of 50cm, the soil is considered to be above the mean highest groundwater level. If the depth > 50cm and the draining area >1,000m², additional testing is requested. 

It should also be noted that the emergency spillway is located at a depth of less than 30cm below ground level.

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