For a Roth IRA, you may withdraw principal contributions tax-free at any time. You only have to worry about tax and penalties if you are dipping into the income (growth).
If you are dipping into the growth, then that part of the distribution will be subject to income tax and a 10% penalty if you are under age 59-1/2. You can be exempt from the 10% penalty using the first time home buyer rule. In the case of a home under construction, the 120 days counts backwards from when you start living in the home (it is complete and you have a C/O and move in).
