Map Editor Help

So I create all sorts of wacky community style games for me and my friends to play, but I've come across a problem I haven't been able to solve yet. I want to activate a trigger depending on a range of inputs. Let's use converting a house as an example.

P1 converts house from P3. P1 now has 1 more house than before, I can track that using the variables, and reward P1 accordingly. However how do I run a check for which player LOST the house, and activate a trigger in response.

I can make it so that when P1 converts a house, all other players get their house count variable deducted by 1, but I can't see how to run a check for which player lost a house, and modify a specific variable depending on that.

I made a scenario where there were kill counts, and each kill earned 50g and deducted 10g from each opponent. I only did it that way because I couldn't solve this. The scenario I'm making now works on capture points, where if a player captures a point they earn resources based on how many points they control. I suppose each capture could run a check for 'objects owned' for each player and modify the variable to equal that amount, but that's clumsy and requires the capture point to be a building players can't build themselves.

Any input is appreciated, thanks very much.