@newgardenroomcompany9676
That's a really good question, and it's a bit of a quirk of the regulations, which set out that you can't retrospectively make an outbuilding Permitted Development by modifying it.
So if you have built something, say, 3.1 metres high and the maximum you would be allowed is three metres, you can't just shave off 10 centimetres according to the law.
However, in practical application, most planning enforcement officers - who presumably would be the people you're interacting with - would accept a modification of the building to come under the regulations. But they have no obligation to to agree to that, it's just that in practical application it would seem pretty unreasonable to demand of you that you remove the building in its entirety and then next day erect it 10 centimetres smaller.
But that is what the law says, so again, this comes back to the theme of making sure your building meets all the criteria for Permitted Development prior to you starting work.
For more information, see our other videos featuring Planning Consultant Mark Doodes.
Recorded February 2023. This content does not constitute formal advice from New Garden Room Company or Mark Doodes Planning.