Well, this depends on what you want to accomplish when you are building houses, here is what you need to know:
1. Build on an expensive adress! :)
By asking around amongst builders you will be able to determine how much it costs per sq.ft (kvadratmeter). This is the first number you need.
The second number is how much you are paying for buildable sq.ft of land. If you buy a lot, there will usually be some regulations that tell you how many percent of that lot you can build and how many stores you can build. Those percent makes for a certain sq.ft of buildable land. Take the purchase price of the whole lot and divide that with the amount of buildable sq.ft and you have the number.
What you do then is to add up the costs of building the house, plus the cost you paid for the buildable sq.ft and compare that to the average sales price per sq.ft in the neighborhood. If there are good margins, go ahead and build, if not, RUN! :)
A hint is that the better the adress, the better the margins. Read more about real building houses and estate development HERE.
Have a great day!