If a home were priced as little at $200,000 then 20% would cover $40k in investments. Homes around here don't sell for the little.

Assuming your percentage is correct and home prices are much bigger it makes it seem like a spectacular investment. Actually too good to be true of an investment.

If the return were 2% or max 5% I could see it maybe not being worth it depending on home prices in your area.

20% is if you insulate your roof and walls, get triple-glazed windows, heat pump with floor heating and solar panels.

That is going to cost in the €150K-€200K range in Europe for a €400K, 150 m^2 house.

20% of the building’s value sounds like a very generous sum for new windows and insulation.

20% is if you insulate your roof and walls, get triple-glazed windows, heat pump with floor heating and solar panels.

That is going to cost quite a bit.