If building is definitely planned, a home savings contract is generally an option. It is then necessary to carefully examine and compare in order to conclude a "good" home savings contract. I would probably do it to secure at least part of the required loan at a favorable interest rate. I would not put everything on the home savings contract, but simultaneously save via a bank savings plan, overnight money, fixed-term deposit ladder (the period is too short for stocks)!
In my humble opinion, building the house with these means is possible, but whether it is advisable depends on many factors...
Other tips: Why are you two (?) already living so expensively in a house? Would a cheaper apartment be an option (availability on the housing market, moving costs), in which you could live as three for a while until a house is built/bought?
Tip: Save as much as possible, simple things, no expensive/complicated bank products, watch the costs! Keep a household budget, watch the costs ;-)!