On 12/05/2021 07:04, Chris Hogg wrote:
On Tue, 11 May 2021 16:32:14 +0100, Chris Bacon
wrote:

I came by an old concrete double garage, which had an apex roof, and I
want to convert it to a flat roof.

https://i.ibb.co/2jgk4gY/DSC00126.jpg

Unfortunately, 2.5m max height applied.

Is this a planning constraint imposed by the local authority, or a
restrictive covenant imposed by a previous site owner...

It's planning.

There is useful information at:

https://interactive.planningportal.co.uk/ (general, interactive, useful
but not exhaustive)
https://www.planningportal.co.uk/inf...3/outbuildings
(sheds, playhouses, greenhouses, general garden buildings and structures)
https://www.gov.uk/government/public...nical-guidance
(permitted development rights)