You need a structural engineer, who may tell you that you first need a soils test by a geotechnical engineer. Building on fill--on a hillside--is not normally a good thing. Was this information part of that provided by the folks from whom you purchased the property? Disclosure is usually a legal requirement.

Thanks. We have an architect, and then did the geotechnical work--this is how we found out. They dug 18ft down and couldn't find bedrock. From looking at survey/aerial mapping, he is estimating the bedrock to be at 50-75ft. Our next door is on 25ft bedrock, and he said it looks like our house is on more fill. His best guess is 60ft. He didn't send off the samples to lab yet since he wanted to ask us if this would kill the project.

We bought our home in 2013; and it is doing well. We were just hoping to add more space. There was a geo report, but did not get this information to the full detail of the depth of the bedrock

Thank you so much for your advice!

Quick question...do we not have the same restriction to add foundation to have to dig all the way down to the bedrock even with a one story addition? I thought since the bedrock is deep, we also cannot add/remodel our existing one story structure without having to re-do the foundation (i.e.: redesign the inside of our existing one story home or add to it on the same story)

Generally you will not be required to upgrade the existing part of the structure unless you're making changes to it. Since this is a bit of a gray area, have a chat with your building department and a structural engineer.

I agree with Virgil. If you have the room to expand the main level, it's a much easier project.

Thank you both. We have the geo survey scheduled, and will decide after we know how deep the bedrock lies. There is room to re-configure our home and add the 650sqft on the main level--it seems this will likely be the most cost-effective way to proceed, right?

Yes. An addition on the main level will likely much easier and less expensive.

Once you have your soils report take it to a structural engineer to see what your options are.

Thanks so much!

have to dig all the way down to the bedrock

Not a necessity at all.

When I had a geo engineer check out a 10,000 year old beachfront I planned to build on, he said no problem, good for up to ten storeys.

We finished our dig, and it looks like our bedrock is likely less than 28ft down (confirmed after the geo engineer went into the hole to check). The location where the dig would be a deeper point, which means our actual home is sitting on bedrock likely 20-25ft down. We will await the final report and take it to a structural engineer to review what our options are...but feel a little better it is not as deep as we initially thought.

That seems do-able. Good luck!