Building a House-Mortgage Interest Relief&Stamp Duty

Recently started building a new house and have paid stamp duty on the site. Is Stamp Duty payable on cost of construction (I'm a first time buyer). Also as i'm getting mortgage drawdowns can i put off starting to claim Mortgage Interest Relief unitl I have the full sum drawndown, because the interest will be low until i drawdown the remainder of the mortgage. Any comments appreciated!

There is no stamp duty on construction. Stamp duty is a transfer tax and the value at the time of transfer is what counts.

Doesn't it depend who is doing the construction? If the site purchase and construstion contract are linked there is stamp duty on the total?