http://simutrans-germany.com/~patches/upload/denkmalschutz-2538.patch
With this patch one can buy houses. Then they will not get removed if the city expands. Also other players can not remove them.
You have to insert something in menuconf.tab to get the new tool general_tool[35].
Building level start from zero. You need to add 1.
And also, you need to add minus accounting.
Also maintaining costs are needed to be taken care of.
So, is accounting maint_building or cst_multiply_remove_haus ?
I think maint_building is too cheap.
And you didn't add 1.
[edit]
This doesn't work after reloading savegame. I recalled this patch.
So, citybuildings of level 0 is free from cost and maintenance cost.