Rush

Hi, I currently own no properties. if I buy a derelict property (£830k) and pay commercial stamp duty rates it will cost £31k stamp duty. Under residential stamp rates it would cost £29k. the renovation works will take about a year, so I am going to buy a small flat for £300k. If I buy the derelict property under the commercial rates, does this mean that I do not need to pay the additional 3% rate of stamp duty on the smaller flat? i.e buy derelict as a derelict property and pay £31k stamp, buy liveable flat in a few months and pay c. £3k stamp. OR buy derelict as residential paying £29k stamp but then have the buy additional flat at 3% additional stamp rate so c.14k stamp? Any guidance over what is allowed would be helpful? Thanks