Capital gains tax and SDLT second home

Hi, my partner and I (unmarried) are looking to buy a house together. It would be my first house purchase but he already jointly owns a flat with his sister. They are planning on selling this within first 9 months of us purchasing our property. Assuming their flat sale completes within 9 months, will my partner have to pay CGT on the sale of his flat? If it sells after 9 months, how much CGT is he liable to pay? The new property we are purchasing together would be my sole residence. Does it make any difference to CGT for my partner if he calls the new property we will own or his existing flat he jointly owns and is looking to sell his main residence? Are we able to have one of our main residences as the new house and his remain at the flat until it sells, given we are unmarried? Assuming he sells his flat within 3 years, will we be eligible for a refund of the additional SDLT we will pay on the sale? Does his designation of his main residence prior to sale have any impact on this? Many thanks

Hi,
You can see the guidance on Private Residence Relief here:
HS283 Private Residence Relief (2024)
Provided the jointly owned flat was his main residence, he would have Private Residence Relief to set against any gain and this could cover all of the gain.  
If he owns a second property, he should conifrm in writing to HMRC, which property is his main residence and from which date, as he can only have one main residence at any one time for Capital Gains Tax purposes.  
There is a Capital Gains Tax calculator which also provides guidance on how to report and pay any Capital Gains Tax here:
https://www.gov.uk/tax-sell-property
Thank you.

Hi, thank you for this. So if he makes the house we are purchasing his main residence once purchased, so his flat stops being his main residence - because it was historically his main residence he is not subject to CGT due to private residence relief? And please can you confirm the SDLT implications of doing this - specifically, are we able to refund the additional stamp duty once he sells his flat (within 3 years) despite him having a new main residence as of when we purchase our property? Thanks

Hi,
That is correct on the Capital Gains Tax part. For advice of stamp duty queries, you will need to contact the Stamp Duty Land Tax team using the link below.
Stamp Duty Land Tax
Thank you.