Greetings.
So, as the title says, the "+X% Life Points for buildings" stat/skill/proficiency is additive, not multiplicative; "+40% hp for buildings" actually just gives "+40 points of health for buildings".
Now this isn't actually a huge issue for Tinker heroes, as they'll get +20 hp per point in the "Engineer" skill, and frankly this is kind of reasonable for that particular skill, however, the "Building" proficiency for heroes seems to use the same additive number rather than multiplicative, which results in a level 50 hero gaining a measly +50 hit points for their buildings.
The racial ability of the dwarves which gives +1% health per building built seems to function perfectly fine, though, so i assume this is just an oversight.
On a side note, the modifier does not affect pre-placed buildings (like the towers you get automatically at the start of most maps) or converted enemy buildings, but it's always been like that so it's nothing specific to the protectors, i just thought i'd mention it.