Stamp Duty Help!

[Francis2020]

Looking at the collective for some guidance please..

I am moving into my partners house she bought it and paid stamp duty. She has about 45% equity and the rest is mortgaged.
For simplicity let's say £1m house with £550k mortgage.

The idea is for me to pay £450k off the mortgage when it ends and the residual mortgage amount of £100k we will jointly pay off.

My question is what do I pay the stamp duty on?
The aim is we will both own 50% each and will have contributed equally.

Some people suggest I need to pay stamp duty connected with remaining mortgage amount £100k.
Others on £500k...

If it helps we are not married.
We want to have it legally clear we both have equally equity in the house should we split etc.

Thanks for the help

[_Rodders_]

Wouldn't it make sense just to pay her back half of what she paid at the time?

Seems a bit nit picky TBH, I don't think I would have even considered the stamp duty element but then I've not been in that situation.

Anything other than selling and buying a new house together and splitting everything down the middle is not going to be a perfect split.