I am knocking out the basement floor of my 110 yr. old house and will be laying a new one after i've installed an internal weeping tile around the perimeter. I have now discovered that there is no underpinnig beneath the foundation, the foundation is simply sitting on the ground below it. Will i need to underpin now? is this costly?

Do you mean you have no footing under your foundation wall?

Since it has worked for 110 years, I would think it ia not necessary to take the chance of construction a footing now. If you have enough area under a wall and the soil has the bearing capacity, then a footing is really no needed, although the codes do not permit it because of the difficulty in writing and enforcing such a method of construction.

In the past, some foundation methods without footings have been approved, but only on a submission case for a specific type of construction. Even a wood foundation can be (and should be) installed without a concrete footing.

If you have a gravity wall (probably quite thick), keep the bottom of the drain tile excavation well inside the inside edge of the wall (1:1 ratio is usually the guideline).

When you pour the floor, make sure you have at least 3.5" of concrete bearing directly on the foundation wall to help prevent any sliding or movement inward. The floor slab is actually an important structural element, but few contractors recognize it.