Commuted sums

In the adoption agreement and conditions a sum of money known as a commuted sum will be requested for the future maintenance of any sustainable drainage systems that will be adopted by the SAB or Highway Authority. These will be calculated using the CSS formula for commuted sums.

The sustainable drainage systems will be adopted by the SAB but not the elements that form part of the highway drainage system where the road is to become publicly maintainable, which in that case the Highway Authority will adopt that part of the sustainable drainage system.

The commuted sum will be calculated using the rates of the SAB to undertake the maintenance, as determined by an agreed maintenance plan as part of the SAB approval. The sum required will be both a condition and will form part of the legal agreement in the adoption of any sustainable drainage system.

Commuted sums for future maintenance

Securing a sustainable funding mechanism for the lifetime of development is a key objective of the SuDS Approving Body (SAB). The SAB has a responsibility for the management and maintenance of SuDS assets after they have been adopted. Therefore commuted sum aims to ensure that the SAB has the resources to cover the upkeep and (where appropriate) the replacement of the assets they have adopted. The effectiveness of SuDS and the associated multiple benefits will rely on appropriate maintenance.

In order to ensure consistency throughout Wales, the use of the industry standard guidance “Commuted Sums for Maintaining Infrastructure Assets” prepared by the CSS (County Surveyors Society), is to be used, to calculate commuted sums for all drainage assets being adopted by the SAB, whether that be through a S38 agreement or a bespoke legal agreement for the life of the developments (60-120 years).

The calculation of a commuted sum includes consideration of:

• the estimated periodic maintenance cost of the asset to the adopted e.g. every six months. The SuDs manual provides useful information on this element.
• its future cost of renewal or replacement (e.g. permeable paving has design life of 20 years, over the lifetime of the development this could lead to 3 replacements).
• the duration over which the sum is required. The Association of Directors of Planning and Transport recommends commuted sums for structures should be calculated to cover a 120 year period and that the period for other items should be 60 years (basically the whole life of the development).
• the effective annual interest rate that will provide a return on the sum invested prior to its expenditure after the effects of inflation have been taken into account (called the discount rate approx. 2.2%).

The use of the CSS guidance is recommended to provide a common understanding for developers and the SAB when entering into either S278 and S38 Highway or bespoke SAB agreements.