Your argument that building a house is 20-30% more expensive now than it ought to be is confusing, especially since you don't intend to break ground for another two years. That gives you plenty of time to interview builders or hire a general contractor, then line up subs for whatever rates you'd like to pay. They could be 30% higher in two years. Or the same, or lower.
What is your budget for building the house? Consider building the basics in 2018 and leaving room to expand later when you have had some time to get your feet under you. Leave room in the budget for overages, which happen frequently on new construction. Don't make yourself house-poor, where you can't afford to maintain and furnish the house after it's built. Another idea would be to save up 20% for 2018, but use anything over that to start building at your own expense and risk, thereby lowering your mortgage costs. You can pour the foundation and drill a well, for example.
You probably want to look at a Construction Draw loan, those require you to pay interest against only the amount of principle that is outstanding. Handy if you run into construction delays and cost overruns (ie, pretty much every house ever built). Some can use the land and any work that has started as collateral for the loan, to keep the rate down. Most draw loans allow you to convert the mortgage to a 30 year fixed after building is complete, saving you the costs of refinancing. Rates are slightly higher than mortgaging an existing home, because the bank is assuming more risk - roughly half to three quarters of a point.
Make sure you protect your credit between now and when you get the loan, and learn as much as you can about the mortgage qualification process.
Many farm families used to do this as their children grew up and married, partitioning off their farm where an extended family lived. I grew up like this, with my grandparent's home partitioned out for two of the four siblings. It was cool growing up with grandparents, an uncle/aunt and cousin nearby.