Well, if you say something about "дом", you have to know "which дом exactly" you are telling about (or, ALL buildings should be of the same kind.).

You can utilize "жильё", if your concern is mostly dwelling houses.

Or just "строительство", which of course will leave unclear, what kinds of objects are subjects to lower prices (implicitly, bridges, roads, etc are being included). In the last resort, please make use of "строительство зданий" (in place of "строительство домов") to refine to the domain of "buildings both dwelling and non-dwelling".

Besides, "стоимость" has synonyms, which are preferable if you want
A) to refer to "whole industry prices" and/or
B) to price that builders had to pay to erect building (as opposed to price that purchaser will have to pay):
These synonyms are:
затраты, себестоимость.

So it would be nice to receive more context, but my suggestions:

"Ожидается снижение затрат на строительство на 20%" - here we should know WHAT are to be built from what had been said before.
"Ожидается, что средняя себестоимость дома [из множества типовых домов, которые строит одна компания] упадет на 20%"
"Ожидается, что себестоимость жилья [в среднем по городу/региону/стране...] снизится на 20%" - you could also say "себестоимость СТРОИТЕЛЬСТВА жилья", but "себестоимость" by itself implies that жильё has still to be created, hence "СТРОИТЕЛЬСТВА" is redundant.