Ancient displaying Health Bar + Selectable through the unseen fog of war

[bright ingot]

Its BaseClass determines that it's a building, building always could be seen in fow

If you insist on using a building

give it a passive ability:

function modifier_ability_test_passive:OnCreated()
    if IsServer() then
        self:GetParent():AddNoDraw()
        self:StartIntervalThink(0.1)
    else

    end
end
function modifier_ability_test_passive:OnIntervalThink()
    if IsServer() then
        if IsLocationVisible(DOTA_TEAM_GOODGUYS, self:GetParent():GetOrigin()) then
            self:GetParent():RemoveNoDraw()
            self:StartIntervalThink(-1)
        end
    end
end

add nodraw until anyone reveals this location

[bright ingot]

so why use a building? replace it with common unit

[quasi slate]

? create modifier and make it untargetable?

and if in radius/attack range remove the untargettable modifier

[median eagle]

is it possible to add a "nodraw" only for one specific team?
No.

You can make it a unit, which is hidden in fog of war

Remember that you're still, in some level, playing in Dota's systems and Dota's rules, even if it's a modification that makes it your own game

Buildings are always visible, by definition, this is how it is in Dota

So if you don't want them to be shown through fog, then you will have to make it a unit that looks and behave like a building - because buildings do not show through fog

[bright ingot]

Let's brainstorm:
Dont put any building on map at first, use some dummy unit instead ( this is for ally display, the dummy unit has same properties & abilities with original building )
And there's a timer working for them, it checks their location visibility, replace them with normal buildings till their location revealed by any enemy team.
CanBeSeenByAnyOpposingTeam the method works for it