Thanks everyone - this is super helpful.
There is also an option of knocking down the existing building and re-building from scratch, including basements. From what we understand the cost of underpinning the existing building will be significant - and it may be cheaper to just knock down the existing (which is currently two old awkwardly shaped flats), potentially retain the facade, and then start afresh, therefore not requiring the same out of underpinning (cost and time).
That said, having spoken to a number of accountants yesterday;
1. existing lenders would expect to be paid back before that could happen.
2. If we then transferred into a limited company, we would have to realise gains thus paying CGT
3. Transferring into a Limited company we would have to pay Stamp Duty
Though this would then mean the whole works would be 0% VAT, as it's complete new builds. Therefore if the increase in value of existing plus VAT savings is greater than the CGT plus additional stamp duty, then it might well be worth it.
The architect we're about to appoint has significant experience in projects similar, so we're aiming on leaning on them a fair bit - but it's the taxation and charges side which as most of you have said is a minefield!!
Quickly realising that the profits that we were hoping for, are certainly not likely - @Temp / @newhome especially given that neither my friend / business partner live in the existing property (both rented out) - and we would have no desire to move into the new builds either - especially for 3 years!
I think for ease of lending/borrowing it would be better for us to assume that we will build and then immediately sell.
@Mr Punter - to your point on being VAT registered - we've been advised by one accountant that if we were to VAT register then we would have to charge VAT on the final sale of the property if it goes to an individual? Is this not the case?
Ideally I think we just need to find an accountant who can handle (and understands) all the taxes and fees that may be payable (incl. CGT, VAT, CIL) - who can then look at all the different building options and advise financially where we will be better off;
ie.
1. Building just to the side (and basement)
2. Knocking down existing building - and developing whole plot
This is hugely insightful, and a great learning - thank you everyone, so far!