Do ashes leak out of stone fire pit

The model building codes provide separation distances based upon radiant heat, construction type, occupancy, fire resistance of the exterior walls and the ratio of openings to solid wall in the building to be protected.

Keeping in mind that the fire separation distances given are in feet from the property line and that adjacent buildings have similar requirements one could use the combination of IBC [2009] table 602 and table 705.9 to determine separation. As a rule of thumb, 60' would be the maximum required between two buildings each located 30' from the property line.

However, I recommend discussing the issue with your local fire official. There may be additional considerations that limit or prohibit the use. The fire official may also be aware of additional environmental, utility infrastructure, or fire service availability factors that increase or decrease the risk associated with a large fire pit.

If you set the forest ablaze, the distance between the pit and the structure will become irrelevant.

You should line it with firebrick or (since you like molding and casting things) castable refractory. Edit: on second thought, stick to hard firebrick - most castable does not like to get wet. Regular concrete is prone to spalling & cracking in direct fire exposure.

Depending on the shape of the "pit" a 2" layer of sand can work, if it will stay put (ie, if the "pit" is rather shallow)