If you want to go deeper than the walls extend you have to underpin the walls.
Lowering garage floor.
For garages it is a little less simple.
In fact for some interior floors it s just a matter of pouring leveling compound out and letting it find its own level.
That s why there should be a moisture barrier between the soil and the floor.
Then install the forms to make an exposed edge that is straight and smooth and pour the floor and hauch in the second step.
That depends how deep the foundation walls go.
If you pour a quarter cup of water on your garage floor it should disappear in about half a minute.
If they extend past the depth you wish to lower the floor it s a pretty straightforward task of removing the floor and excavating the dirt and stone to the depth you want.
We are in the midst of renovating all the landscaping and masonry work in the front of the house also a very steep slope and the landscape architect has made the suggestion of lowering of the slope of the driveway by lowering the garage floor about 2 feet.
Lastly a layer of gravel is poured leveled and is covered by a vapor barrier and reinforcing grid readying it for the newly poured concrete floor.
First dig the trench and fill it asap with concrete to say a foot below the finshed floor or where you want the lower protion of the haunch.
Install dowel rods 3 4 oc and leave the top of the concrete rough.
If your garage floor is sealed the water will eventually evaporate.
Apply leveling compound in certain spots.
Leveling compound is the classic and the easiest to fix a floor that is uneven.
Concrete floors can t be sealed from the underside.
To lower the floors to give an extra18 inches you would have to excavate a minimum of30 inches to allow for hardcore insulation concrete this will take you under the footings sugest you abandon this idea regardsterry.