re: Building A House - Where To Start?

I think my main concern is financing. Is that true that you should have 20% of land+home to get started? And how do you know exactly what and how much to ask for? Or do they say, "Ok, we approve you for $300,000. You draw it as needed." Then whatever you don't use, it doesn't go on the mortgage and if you go over, you pay out of pocket?