Guidance

Using unbound incinerator bottom ash aggregate in construction: RPS 325

Published 20 May 2026

Applies to England

This regulatory position statement (RPS) does not change your legal requirement to have an environmental permit for a waste operation and groundwater activity when you use unbound incinerator bottom ash aggregate (IBAA) in:

• building a road, cycleway or pathway sub-base

• building a structural platform

• pipe bedding

However, the Environment Agency will not normally take enforcement action against you if you do not comply with this legal requirement provided that:

• your activity meets the description set out in this RPS

• you comply with the conditions set out in this RPS

In addition, your activity must not cause (or be likely to cause) pollution of the environment or harm to human health, and must not:

• cause a risk to water, air, soil, plants or animals

• cause a nuisance through noise or odours

• adversely affect the countryside or places of special interest

1. Activity this RPS applies to

This RPS applies if you use unbound IBAA in construction as a substitute for non-waste aggregate. It includes storing IBAA associated with that use. This also applies to IBAA in hydraulically bound mixtures, where IBAA is mixed with water and a binder such as cement.

2. Conditions you must comply with

You must:

• only use IBAA which has come from processed municipal waste incinerator bottom ash

• only use unbound non-hazardous IBAA, waste code 19 12 12

• comply with BS EN 13242 or any revision of that standard

• meet the limits and conditions in this RPS for the total tonnage of IBAA if mixed with non-waste aggregate

• meet the limits and conditions of this RPS for each construction activity carried on at the same development

• meet the limits and conditions of this RPS for any temporary construction

2.1 All uses

For each delivery of IBAA to the site of use, the end user must get in writing from the supplier, the:

• date of delivery

• name, address and contact details of the IBAA producer and distributors (as applicable)

• quantity supplied (by weight)

The end user must keep records for 2 years to demonstrate compliance with this RPS and make these records available to the Environment Agency on request.

You must:

• only store IBAA in the open

You must not:

• store IBAA for more than 6 months

• store more IBAA than you need

• store or use IBAA within a groundwater source protection zone 1 or 2

• store or use IBAA within 50 metres of any spring or well, or any borehole used to supply water

• store or use IBAA within 25 metres of a water course

• use IBAA below the water table

• use IBAA underneath any building occupied by people

• use IBAA on any landfill whether historical, closed or operational

2.2 Building a sub-base for a road, cycleway or pathway

You must construct the surface of a road, cycleway or footpath with an asphalt or other low-permeable material.

You must not:

• install a sub-base using IBAA of more than 40 metres wide

• install a sub-base using IBAA of more than 0.7 metres deep

2.3 Building a structural platform

You must not:

• store and use more than 25,000 tonnes of IBAA in total to build a structural platform

• build a structural platform using IBAA that is on average more than 1 metre thick

• build a structural platform that has less than 95% coverage with a low permeability surface or building

• build a structural platform using IBAA within 25 metres of a surface water body

• build a structural platform unless it complies with the restrictions set out as follows:

Distance to surface water body (metres)	Maximum tonnage (tonnes) (Dry)	Volume after compaction at 1.7t/m3 (m3)	Maximum surface area of a structural platform (m2)
25 to 49	4,420	2,600	2,600
50 to 99	6,800	4,000	4,000
100 to 149	13,600	8,000	8,000
150 to 199	20,400	12,000	12,000
200 or more	25,000	16,000	16,000

2.4 Pipe bedding

You must use a low permeability surface above the pipe-run with adequate falls to prevent standing water.

You must not:

• store and use more than 510 tonnes in total of IBAA as pipe bedding in any single construction project and pipe run

• use IBAA as pipe bedding more than 0.3 metres deep in a trench more than 2 metres wide

• use the IBAA as a drainage medium

3. Things to note

This RPS relates to activities that the Environment Agency considers are potentially suitable to be an exemption for a waste and groundwater activity under the Environmental Permitting (England and Wales) Regulations 2016. A change in law would be required for this.

A surface water body is a relevant territorial water, coastal water or inland freshwater (as defined by the Water Resources Act 1991).

A structural platform is an engineered base for an overlying construction feature, for example a:

• building
• car park
• retaining wall
• storage tank
• road or pavement

A structural platform includes IBAA used in engineered fill layers.

A sub-base includes IBAA used in the capping layer below the sub-base.

Pipe bedding is material laid in the bottom of a trench that has been excavated for drainage pipes or other pipelines.

4. When you must check back

The Environment Agency will review this RPS by 1 July 2029.

The Environment Agency can withdraw or amend this regulatory position before the review date if they consider it necessary. This includes where the activity that this RPS relates to has not changed.

You will need to check back from time to time, including at and before the review date, to see if this RPS still applies.

This RPS remains in force until it is removed from GOV.UK or is otherwise identified as having been withdrawn.

You can subscribe to email updates about this RPS. These will tell you if the RPS has changed and when it has been withdrawn. Use the ‘Get emails about this page’ feature on the RPS publication page. 

5. If you cannot comply with this RPS

If you operate under this RPS but can no longer comply with it, you must:

• stop the activity to which this RPS relates
• tell the Environment Agency immediately by emailing enquiries@environment-agency.gov.uk with RPS 325 in the subject

6. Contact the Environment Agency

If you have any questions about this RPS email enquiries@environment-agency.gov.uk with RPS 325 in the subject.