My wife and I were ecstatic over recently locking in on a 3.5% conventional fixed rate home mortgage. We learned that LTV which means Loan to Value ratio AND credit score are the sole determinant of the interest rate you get. LTV is essentially the same thing as how much of a down payment your make. For example, if your LTV is 75% you put 25% down on your home which means you have to pay no points for the lowest rates available.
If your LTV is too high, then you could pay as high as 3 points which would mean you would pay 3% of the amount you are financing after your down payment. Also if your credit score is too far below 660 your interest rate will be closer to 4.5% which can add over $100 a month to your monthly payment for a home priced around $200,000.
Different combinations of LTV and credit score determine how heavy the ballast of your monthly mortgage payment. Strive to pay as much down and get your credit score as high as possible. This can save hundreds of dollars on your monthly payment!
Also do not lock in too soon as quote guarantee agreements have expiration dates, usually around 45 days. Get a firm construction completion date from your home builder before locking in, because if you exceed the expiration date then you could be stuck with a much higher interest rate than the expired lock-in rate.