In message , Vortex
writes
I have 3 sheds, approximate ages 10, 20 and 30 years and in various stages
of bio-degradation.

I would like to replace them with one robust over-engineered "mother of all
sheds".

[...]

Local council web resources are very unhelpful. When is a shed not a
shed...but an "outbuilding"?

Http://www.odpm.gov.uk/stellent/grou...nts/pdf/odpm_p
lan_pdf_606151.pdf

Doesn't matter if it is a shed or an outbuilding. As long as it is
within permitted development limits, it will be exempted.