I would think you'd need to contact a local contractor, as well as a structural engineer, in order to receive even a ballpark quote on this sort of job. Maybe talk to your neighbors who have done this, and ask them what they paid, how involved the process is, etc. I could tell you what it would cost to do this where I live, but I can almost 100% guarantee you it would be drastically different from what you would pay there. You would also probably need to contact your city government with regard to pulling permits to see if this is allowed - and if you're in a historic district, you would need the historic preservation group's OK, too.

As a clarification, our house is 120 years old and the existing foundation is showing it's age. I doubt it's going anywhere, but adding to it is questionable at best. The idea of the mushroom footings and columns is to avoid adding stress to the existing house. The new piers would replace existing piers along the back wall of a one story addition which started out as a porch. My initial description is incorrect. Thanks mightanvil for pointing that out. There would be four mushroom cement blobs buried 4 feet under supporting typical 10" cement columns along a 14 foot wall. The columns would support 4' lengths of lally (exposed to the elements), then 4X4 wood columns would run through the first floor wall and carry a beam for the second floor 2x12x18 24" o.c. joists. The 4X4's would continue to the roof line and support another beam for the flat roof 2x10x18 16" o.c. joists. Maybe those are called rafters, but it's a flat roof so they might be joists. The structural engineer designing this is the architect, designer, and owns the contracting company. It's a design/build outfit that's been around a long time, and the city inspector relies on his stamp of approval. I'm concerned about minimal requirements. For instance, we had some options to upgrade from 2X10's to 2X12's for an 18 foot span. I thought that was a good thing. So as part of a $3,000 work order change I was surprised to see the plans had the 2 X 12's at 24" o.c. After he explained that his plan met Massachusetts building code, he wants another $300 to make them 16 o.c. and thinks it's a needless change. If he didn't have our 1/3 deposit ($8k) and a ($3K) change order to beef things up, I'd move on. I've only seen piers supporting decks, and some houses along oceanfront properties. Been searching the building codes and haven't seen much written about this design. Code is linked at end of posting. Any assistance or words of wisdom appreciated. I will pay attention to the bracing within the walls of the 4x4' columns as suggested above by early1. Here is a link that might be useful: Mass Building Code online

Your max is the lowest bottom end potential cost. It will more likely be 30-80% more than that for something average in build quality. Depends on if you are in Kansas City, MO, or Alexandra, VA. That is if the foundation and garage framing can hold up a second floor, and the top plate heights at all line up with the home. And the home's main drain is on the same side of the house. Most likely one of those assumptions is not the case. You might have to tear down the garage and completely start over as a new build. And pay for a new sewer tap. If allowed. If the main drain is in the wrong spot, the project will never be feasible to do.

Houssaon, Thank you for the suggestion. I agree it wouldn’t have too much impact. The upstairs double hung windows are oddly spaced extra wide apart externally to fit on both sides of the bed in the upstairs MBR. Matching them would double up on the odd spacing and be somewhat costly and disruptive. The bay is southeast facing and brings in some nice light into the great room during the morning. A porch roof will create shade in late morning but should provide a better tie-in to the house.