As I read it, the higher rate applies:
Quote:
"If you’re married or in a civil partnership, buying a property and your spouse or civil partner already owns a property you may still be liable to the higher rates. But you may be able claim a refund if they then go on to sell it"
However, if you'll actually be moving into the new house (rather than keeping it as a second home or letting it out), this would then apply:
"You won’t pay the higher SDLT rates if the property you’re buying is replacing your main residence and that main residence has already been sold.
If you buy a new main residence but the sale for your previous main residence is delayed you’ll pay the higher rates as you own 2 properties.
But you can get a refund for the amount above the normal SDLT rates if you sell your previous main residence within 3 years"
See here:
https://www.gov.uk/guidance/stamp-duty-land-tax-buying-an-additional-residential-property