An Update to Permeable Pavement Design Standards: What You Need to Know

In March 2026, an important update was made to the British Standard that guides the design of modular permeable pavements. The previous document, BS 7533‑13: 2009, has been formally replaced by BS 7533‑103: 2026 – Design and installation of modular permeable pavements – Guide.

This update reflects changes in best practice and provides clearer, more up‑to‑date guidance for anyone involved in specifying or designing permeable paving. It also brings permeable pavement design into closer alignment with the wider BS 7533 suite of standards.

What’s Changed?

The new BS 7533‑103: 2026 introduces a number of key improvements and clarifications, covering both the structural and drainage performance of permeable pavements. These include:

• Updated methods for calculating pavement strength and durability
• More detailed guidance on the specification of materials and system components
• Greater clarity on the make‑up of System A and System C pavements performance – how water should flow through and be stored within the system

It also aligns more closely with other standards in the BS 7533 series, making it easier to apply consistent guidance across different paving projects.

Overall, the revised guidance provides designers with more detail on:

• The different components that make up permeable pavement systems
• Clarity on materials that can be used
• How designs should vary depending on traffic levels

The aim is to make designs more robust, consistent, and easier to justify.

A New Approach to Drainage Design

One of the most significant updates relates to how rainfall and drainage performance are assessed.

The new standard now follows the recommendations set out in BS 8582, using the FEH (Flood Estimation Handbook) rainfall intensity formula instead of the previously used FSR data. In addition, recognised drainage modelling software is now required to assess performance and ensure the system’s available storage is used as efficiently as possible. This replaces older rainfall data that was used in the past.

In simple terms, this means permeable pavements are now designed using more accurate rainfall information, helping to ensure the system works efficiently and makes the best use of available water storage.

What Does This Mean for PermCalc?

Because of these changes, the PermCalc software has been withdrawn and can no longer be used to design concrete block permeable pavements in line with the new standard. While PermCalc supported earlier versions of the standard, it does not use the updated rainfall methodology or modelling approach now required.

Still Need Design Support?

If you need assistance with designs that comply with the new standard, support is still available. Our technical team can help calculate pavement requirements in line with BS 7533‑103: 2026, using the correct methods and assumptions.

For design support, please contact: projectdesigner@brett.co.uk

We’re happy to help ensure your permeable pavement designs meet the latest requirements and best practice guidance.