You probably will want to build on the lot with the modular, that way you might be able to use the same septic and water hookup and avoid any potential impact fees (big issue around here but may not be down there). I am pretty conservative and feel you never know what may happen it the future. For that reason I would try to keep the lots separate and get a loan on only the one with the house that is going to be built on. This means if something happens you could always try to sell the lot off and keep the house, or vice versa. It will cost you a little more in taxes, but try to keep the lots with separate tax id's. Otherwise the state may combine them as one lot and then refuse to let you separate them later on. By doing this, the owner can lose 75% or more of their value on the additional lot. The counties can and will change policies at their own discretion, often with no notice but only by changing their interpetation of the law. You may not care, but your children may. We have been paying up to $3,500.00 or more to remove old mobiles, so if you can get rid of it free do it. If the lots are big enough, there are times it can be smart to leave the old mobile on and change its status to storage. This can create a buffer between you and sensative areas and allow you to build closer to them then if you remove the mobile first and try to build later. If you run into problems getting your permit, try to hire a consultant before just doing what the town or county officals recommend. They are often wrong and don't always know their own code. Hopefully your small town is receptive and it is easy. Small town politics can make building the easiest thing or hardest thing around. Good Luck.