Flaperon google multiple dwelling relief
Quick google brings up https://www.bdo.co.uk/en-gb/insights/tax/tax-support-for-professionals/granny-flats-remember-to-claim-sdlt-multiple-dwellings-relief
Our conveyancer, and their entire firm hadn’t heard of it, but were great at sorting it out, they have a duty to make sure you pay the right stamp duty so we’re actual open to being sued as they didn’t know about this…
I believe it could also apply if the second dwelling is separate as long as it meets other criteria. I believe you have to buy all properties in the same transaction to get the exemption, so sounds like it wouldn’t apply in your circumstances.
I’m not a lawyer though…