It's not laws in the UK, but you do have to get planning permission. And while you can buy old and/or expensive houses in this much land, it's rare for new builds. First of all, most houses are built commercially to make a profit and they make more profit the more houses they cram onto the site. Second, the planning committee at the local council may not like a lot of land being used for one house. They have local forward plans to meet which include how many houses a year are wanted to be built in a particular area, and land to build them on is one of the constraints. Building your own house on land you own is probably the easiest way to do this in the UK, but you do still need to get planning permission from the local council. If you build it without they can force you to demolish it, and many councils have done precisely that.