ah, I see what you mean. The second property SDLT rates.
Then, yes, as Jamaba says, you will pay the higher rate.
If you complete the sale of your first house within 18months you will be able to claim a refund.
The problem, here for you is that you will make up one of a very small number of people who might get caught out and excluded from the policy buffers designed to keep the higher rates off those who just want one main residence. The 18month limit is arbitrary and is done to allow for sticky sales. Its not done to allow time for development. If you’re quick you could well get your development completed, move out, move in and sell within 18months. You might consider moving out and renting during the build.
If you were buying bare land (non residential) you wouldnt have a problem. Unfortunately gardens are classified as resi property, and the classifications arent changing.
there’s more to read here:
https://www.gov.uk/government/consultations/consultation-on-higher-rates-of-stamp-duty-land-tax-sdlt-on-purchases-of-additional-residential-properties/higher-rates-of-stamp-duty-land-tax-sdlt-on-purchases-of-additional-residential-properties