If someone is obtaining wealth to build a house is zakaat compulsory in this case.
Assalaamu Alaikum Wa Rahmatullah,
الجواب و بالله التوفيق
If the monies already saved have reached the Nisaab and one year has passed over this nisaab then you will have to pay zakat on it.
Also, if you already possessed a nisaab besides the wealth you are obtaining to build the house, and this wealth was still in your possession when your zakat year ended and you were paying zakat, then you will include it in your total and pay zakat on the entire amount.
If however, the wealth for building the house came into your possession but was used up before the date arrived when you paid your zakat at the end of a year, then you will not pay zakat on it.
And Allah Knows Best.
Mufti Waseem Khan.