In building terms the oversite refers to a layer of concrete that is used to seal the earth and create the ground floor of your house.
Concrete oversite floor.
At the time many ground floors of houses were formed as raised timber floors on oversite concrete with the space below the floor ventilated against.
So placed to prevent disturbance of the ground below to provide a relatively even and firm surface for the placement of the next layer and to keep out ground air and moisture.
These are laid on top of the inner leaf of the walls sitting on the dpc but with no mortar bed.
The 440mm gaps left between the beams are then filled with standard size foundation blocks until the entire floor is one flat slab.
Like everything it is a very important step as it is what you will be essentially walking on when the extension is built.
From the early part of the twentieth century it became accepted practice to cover the site of buildings with a layer of concrete some 100 mm thick the concrete oversite or oversite concrete.
Something that is left out missed or forgotten.
An underlayer of concrete below a slab or other flooring.