Some useful documents

Environment Agency H4 Odour Guidance - The document that details numerical odour standards for odour assessment

2002 Draft of H4 - This was a much more extensive document and has some good background information

Defra Local Authority Odour Guidance - useful guidance covering many aspects of odour assessment - note that Defra withdrew this guidance in September 2017 but no replacement has been issued

IAQM Guidance on Odour and Planning - the only guidance relevant to planning

Defra Guidance for Commerical Kitchens - contains the methodology used for a risk assessment of odours from restaurants/takeways - note that Defra withdrew this guidance in September 2015 but no replacement has been issued