Permitted development for back garden woodwork workshop?

[Shlumaan]

Hi guys, hope someones got some good grasp of the building regs and permitted development and how they may apply to my scenario. Just had a company to lay a 7x3 meter concrete slab at the rear of my garden bordering two neighbours.I'm planning to build my woodwork workshop on top of the slab and need to figure out how to maximise the size I can build on top of this pad, and how to get the maximum internal head height while sticking to the regulations. The slab is 20cm in depth, 10cm below the garden lowest point and 10cm above the lowest point. However, it was obvious from the start that our garden level is very uneven, and that im some parts the slab is actually below the surrounding soil level especially around the fences where the neighbours garden begins at least 20cm above the pad level. For that reason I'm thinking to build a retaining wall to keep the soil at bay. I'm attaching some photos to help assess my first question which is, what would be a viable point to measure the 2.5m external roof height as per permitted development within 1m boundary of the fence? Do I take the lowest part of the garden as a reference, the highest point or possibly measure it from the slab itself? Is it a matter of being sensible or is there a hard and fast rule to go about this? In an ideal world I would like to be left with 2.4m internal height, after deducting the wooden base and cold roof insulation which will probably amount to 30-40cm. But if my ground level is the lowest point and my slab is 10cm above that, then I'll be left with 2m head height. If I measure from the slab then I might get 2.1m and if go from the higher points I might be able to gain myself another 20-30cm. That of course will raise the chances of a neighbours complaint as the outbuilding will rise well above the fence and possibly a follow up inspection ,a position which I really don't want to find myself in.

Any thoughts, advice, or similar experiences to share will be most appreciated.

Shlumaan

 

[MH731Z]

Given the unevenness of your garden, look for the highest natural ground level around the slab. Measure the 2.5-meter height limit from this point. Using the slab itself as a reference may not comply unless it aligns with the highest natural ground level surrounding it.

P.S I'm not a building expert so my advise is just based on what I've researched

 

[MH731Z]

just thinking about this further, if you want to play it safe consider the average ground level around the slab as the reference point then go 2.5 from there

 

[Shlumaan]

Thanks for your thinking. From the photos, would you say the average is above the slab level? How do you then 'prove' it if you had a visit from the council? You said you researched it, could you send a link to any good source that addrsses this ambiguous scenario? Thanks

 

[big-all]

before and after pictures
"tide marks" on fence panels/posts/brick/path any other structure that can show a datum point touching or as near as possible to be counted

 

[Shlumaan]

Is it worth the risk? I'm thinking the damage of potential inspection and then trying to prove how it used to be 10y back etc. Would it not be a better idea to apply for bigger sized shed? Will they approve 2.7m high and 20sqm floor size? That will sort me out no matter from where I measure the height. I don't mind paying £200 for an application and to wait a few weeks if you think no reason to reject it. Any thoughts?