Get the under 2.5m rule if within 2m of a boundary.
Not sure on the size restrictions, says no more than 50% of land to be occupied by outbuildings when compared to original house. My place had a double garage in the 60s part of which was subsequently converted to living, no planning but the wiring is 60s ish, so similar era.

Question. Does the land mean the whole plot or just the back garden? Does it make any difference that the 60s garage is now converted into the main house?

I’m sure it is well within size anyway even taking into account the double garage.

Bonus question. Is there any restriction on materials if built on a boundary? Have seen some mention of fireproof materials on forums.

Bonus question 2 if the shed has a covered over area that is open on 2 sides (to put a table and chairs under) does that count as part of the structure? It would share the same roof but not be enclosed.

Land is the whole plot.

The fire proof thing is a building regs issue. Not planning. If it is bigger than 15m2 and within 1m of a boundary then it must be non combustible. If it is bigger than 30m2 then it’ll need full building regs sign off.

You aren’t allowed a veranda and all under permitted development. That is unlikely to be enforced though.

The original house refers to how it was on 1st July 1948 or when it was built if its after that date.
Its 50% of the area of the land around the house (not just rear garden).
In theory it should be made of mostly non combustible materials if its close to the boundary.

Land is the whole plot.

Not according to this lot.

http://www.sheds.co.uk/planning-permission/

Besides, if it was the whole plot, then if you had a garden the same size of your house, then you could cover in one giant shed. Defies the purpose of the rule I think?

They’ve got it wrong sadly.

https://interactive.planningportal.co.uk/mini-guide/outbuildings/5

I take it treated timber aka tuin type cabin would classify as combustible?

Would the 15m2 be the ‘enclosed space’ or include the veranda?

Tbh could do without the veranda if it causes issues and do something separately non permanent with removable shades and a frame

This would replace an existing timber shed in the same spot

Verandas are not PD although its not clear if thats only if they are attached to the house.