Can anyone help? I have a query regarding stamp duty. My partner bought a house 2 years ago. I’ve now sold mine and want to go on his mortgage so we own the property together. It’s worth £400,000 and the current mortgage is £200,000. I intend to put £100,000 towards it from my house sale, thus halving the mortgage. Can anyone tell me what stamp duty I would be liable for to do this?