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.

The house is actually about 2000 sq ft as is, and a fully remodeled interior. The goal is more so to add some height because the neighbor hoses on both sides tower over it. Also to add a wanted open loft area. I think this is a case where you're making a mistake. You don't need to match the other houses in the neighborhood and in fact being the one different house with 2000 square feet could be a huge selling advantage when the time come if someone wants a one floor house within the neighborhood.

Yes, it depends on the person you hire. A “handy” type person that isn’t licensed and insured will end up costing you a LOT more in the end after the fines for no permits and inspections happen. And the inevitable seismic retrofitting required by the scope of such a project that your handy person doesn’t have a clue about is going to be an issue with the city as well. Red tags for days. As a realistic start to home project costs. Double it as a start for your location differential. https://www.remodeling.hw.net/cost-vs-value/2018/