Is zakat obligatory on saved money designated for building, after it was previously subject to zakat, then the saving stopped for the purpose of buying land and building on it, then saving resumed along with borrowing money for building?

Money saved for any purpose is subject to Zakat (definition) if a full lunar year (hawl) has passed over it, and it reaches the nisab (definition) (minimum threshold) by itself, or when combined with other cash or trade goods. Therefore, the money saved by both of you, as well as the money borrowed by the questioner from the bank, is subject to Zakat if the conditions for its obligation—namely, the passing of a full lunar year and reaching the nisab—are met. It should be noted that Zakat becomes obligatory only if the share of each individual separately reaches the nisab, not based on the total accumulated amount.