i dont know as much about pigeons as most as the guys on here do, but i do know about construction
dig down about 24inch, 600mm in modern measurements forming a square hole 12''wide, pour in concrete to fill the hole, and repeat every 6 feet along the length of your loft or approximate measurements to fit the length
depending on the width you will want the same approximately 3 feet apart
on top of the concrete pads which must be level with each other, build brick pillars 2 bricks wide per course, on top of the brick pillers which want to be approximately 2 or 3 bricks high (depending on the finished height of your loft) place 4x2 tanalised timber to sit your loft floor on...if you can get hold of some cheap railway sleepers use those instead.......... the loft will never move unless you have built on a bog
becarefull how you site your loft re your neighbours, some councils insist on planning permission for a pigeon loft
kev