When I have building hp bonuses from e.g. the Engineer skill or the hero Building Proficiency, and I build a "capital" building (e.g. Empire Palace or Dark Dwarf Furnace), I receive the expected bonus to the building hp (e.g. 4100hp with a lvl 100 building proficient hero with 100 engineer skill & 5 points in the Durable Builder perk). However, when I upgrade it to level 2, the building hp is always 900, no matter what. Even the higher levels do not benefit from the hp bonuses as they should, but are what they would be if I had no bonuses. This also happens with other buildings on upgrade, such as The Swarm Hives and Daemon Summoning Towers and Lightning Spires.
Additionally, the engineer skill (+3% per skill point) and hero building proficiency (+1% per hero level) bonuses to building hp seem to multiplicative, not additive. But I'm guessing that isn't a bug though.
I love your work and look forward to the release of 0.8.9