How do you strengthen an existing foundation for an addition?
There are a variety of methods and materials to structurally upgrade crawlspace and full basement foundations to carry the weight of a second-story addition. Some simply require more piers and additional floor beams or joists; others may need to be retrofitted with shear walls in the corners. It's not easy (or inexpensive) work, but it rarely can’t be done.
It all depends on what you are trying to accomplish, to have your plans approved by the local building department and be granted a permit for the addition they will have to be reviewed and approved by an engineer. The engineer will be able to tell you everywhere you need footings, pads, re-enforcing of existing walls and load barring posts.
The engineer will spec the sizes and placement of all things listed above. Your contractor will be able to price you the cost of all that off of the drawings.
It is very dangerous to try and do this yourself, especially when it is holding up a central part of the home. More than a few people have been injured or killed trying to re-enforce foundations with a house above them and a collapse happens.
There are other things you need to consider when doing an addition to a home, you have to re-mediate the weepers system and waterproofing not just the new addition but also the tie-in between the old and new building.
Spend the money on an engineer and you will be guaranteed to have a well re-enforced foundation no matter what the size or scale of the addition will be.
Village Builders Inc.