You still end up owning the house, so it's more of a conversion in savings. And you'd probably be paying it off over time, with the help of ongoing income, if the mortgage rate isn't super high.
Land depends on where you go, no comment there, but keeping up payment on your current place during construction just means you end up paying for 21 years of housing on a 20 year mortgage, or 31 years of housing on a 30 year mortgage, not a big deal.