1. Can the council planning department please document for me where they measure the height of an outbuilding (CLASS E) when it is a) within 2 meters of a boundary & b) on a sloping ground.

Supporting information:
Extracts from Section E, Page 43 of Permitted development rights for Householders - Technical Guidance (September 2019) https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/830643/190910_Tech_Guide_for_publishing.pdf
a) The height of the building, enclosure or container should (recommendation) be measured from the highest ground level immediately adjacent to the building, enclosure, or container to its highest point.
b) If any (unconditional) part of the building, container or enclosure is within 2 metres of the boundary of the curtilage of the house, then the height limit for the total development is restricted to 2.5 metres if it is to be permitted development.

Additional References:
The Town and Country Planning (General Permitted Development) (England) Order 2015
https://www.legislation.gov.uk/uksi/2015/596/schedule/2/made
Class E of part 1 of schedule 2 of the General Permitted Development Order 1995 as amended states outbuildings within 2 metres of a boundary cannot be more than 2.5 metres high.

Planning Portal Mini Guide:
https://ecab.planningportal.co.uk/uploads/miniguides/outbuildings/Outbuildings.pdf

The Council measures the building in accordance with the guidance which is included above as supporting information. If intending to build an outbuilding then the Council advises them to submit an application for a certificate of lawfulness.

If the building has not been built and plans are provided with a certificate application then the Council measures the height of the building on the plans. If the building has been built then the Council measures it on site.