CGT and Stamp Duty - derelict property and liveable property

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

Hi Rush,
Please contact Stamp Duty Land Tax helpline to help answer your questions raised. 
Contact details here:
Stamp Duty Land Tax
Thank you.