Building in a side garden

I came across this recently and thought I'd make a post of it in case anyone is in a similar situation. Someone I know had a big side garden and realsed it would make sense to build a house on the land and sell it. His solicitor now informs him that he is liable for CGT on the income he generated from the new house because it is not his principle residence and he never lived in it, They only way to avoid this is to move into the new house yourself(which should be stamp duty exempt because it's a new build) and sell the old house which is not subject to CGT because it was the principle residence and was occupied by the owner. He got a 40k tax bill.

"Someone I know had a big side garden and realised it would make sense to build a house on the land and sell it. "

But would someone not sort of think that there might be some sort of taxation/financial implications on what is essentially a business venture? Is it not a bit late in the day having done the deal to start worrying about tax?

mf

Yes it is, thats why i put the post up, in case someone repeated his mistake.