How do you start from scratch and square up an area to build a porch or other building when there is no available reference point to start

Would this porch be up againist a building? Use stakes and string to lay our your first straight line. Usinf the 3/4/5 method you can then square the second string to the first string. How to Design, Plan and Build a Deck (http://www.lowes.com/lowes/lkn?action=howTo&p=Build/BldDck.html)