Ok I'm not the greatest with math so I'm kind of stuck here. The last place we purchased, I wasn't able to go halves with my investing partners (my brother & sister in law) because he had maxed out his borrowing capacity. So I put it all under my name, which has now tipped me over the land tax threshold. We all expected this to happen so it's no surprise and we had always planned on splitting this expense fairly. However I'm not quite sure how to divide this. Is it just a matter of taking his percentage of my overall portfolio? And if so, do we use the land value (LV) or real market value? E.g. imagine the threshold was 1m, and my total land value was previously 800k. After the last purchase, it increased to 1.1m (but 1.3m in real terms), which puts me over the threshold. If we had gone halves, it wouldn't have put me over, but it has. What % of my land tax should I allocate to him? Three options: 1) It seems to me that if he owns 50% of the new 300k property, that's 150k (going by the land value, not the real value, which is the correct value to use right?). So therefore, that's 150k out of 1.1m, which is 13.5%. So 13.5% of my land tax is owned by him. 2) OR, do we say because I only crossed the land tax threshold by 100k, therefore he is only liable for 100k out of 1.1m, which is 9%? 3) In some ways it seems unfair on him. Say he had a 100k portfolio and I had 900k. He's now paying a disproportionately high amount of tax if we split this new house's impact by 50/50. So if we were to consider his side, 150k + 100k, that's well below the threshold and therefore he shouldn't have to pay anything. Which one is right? I'm sure I've missed something here... #1 seems the most obvious way but #3 gets me thinking that maybe I'm looking at this completely wrong.