Stamp Duty to build on site I own?

Hi,
I bought a house with a side garden I hope to build on. I paid Stamp Duty on the purchase but wanted to double check will I have to pay stamp duty when I go to build? I'll be selling the house minus the garden, then proceeding with the build. When buying a site you've to pay stamp duty on the value of the site and the value of the proposed build, but what about my route?
Thanks

If you already own the site then there is nothing to stamp.

Building contracts are not stamp able in this context.

Would the building site not need a new folio no when applying for PP and if not, will it not need it when the house is sold and the site as it stands now needs to be divided?