Stamp duty when buying in to partner's property

glenrothes

I'm struggling to find good resources online to self-educate ahead of speaking to a solicitor. Does anyone have insight on the below, or anything related that is worth knowing? Thanks!

As far as I understand when buying into a partner's property you must pay stamp duty (or Land and Buildings Transaction Tax (LBTT) as in Scotland) on the amount bought into. This is both as the cash amount given, or the amount of debt transferred as their responsibility.

When buying in to a mortgage with £350k remaining, and buying half (£175k) of this (some in cash, some in taking on responsibility for the debt), then LLBT would be payable on that £175k. For Scotland this would be £0 up to £145, then 2% on the remaining £35k.

1) Is this accurate?

2) How does this work if the person buying in later makes overpayments? If one person overpays beyond their half, what happens in relation to tax?