SI 2017/365 The Planning (Hazardous Substances) (Amendment) Regulations 2017 (link)
These Regulations amend the Planning (Hazardous Substances) Regulations 2015 (link), which list the hazardous substances and quantities of those substances which are controlled under the Planning (Hazardous Substances) Act 1990 (link). Those Regulations also set out a process for obtaining consent for controlled substances.
These amending Regulations serve to clarify a rule about how controlled quantities are calculated where two or more hazardous substances are present together but individually the amounts of those substances fall below the usual controlled quantities. The rule must be applied in relation to each of three hazard groups to assess whether such substances nevertheless require consent.
Note that the Planning (Hazardous Substances) Regulations 2015 apply in England only.
Source: HMSO