Are you financing this yourself, out of pocket? Or are you seeking a mortgage from a bank or credit union? Do you need a building permit for approval prior to construction? Do you plan to sell the property in the foreseeable future? If the answer is yes to any of these questions, you need to contact your local building official to see what code-approved options you have for foundations.

Drilled piers or treated post footings may be a possible option to a continuous, poured footing.

Wondering about the condition, stability and health hazards of a former horse stable, have you considered demolition (saving and reusing selected timbers and siding), and building from scratch?

Financing out of pocket, trying to legally do as much work as possible on my own to save money. Don't plan on selling until I'm dead haha. For further cost savings I don't want to demo and start over. I've read a lot about stilts, pilings, piers, treated posts...etc as options that involve digging holes at the corners of the building and every 5-8 feet to set the supports. My math came out to 25 total supports for the perimeter wall and under the floor frame work.

I'm wondering if that's enough support and how deep I need to dig for Evergreen,CO to set the posts or rebar-enforced concrete-filled sonotubes?

A frost protected shallow foundation can only be used for buildings that will be heated. Before talking to the local building inspector find out as much as you can and expose and photograph the footings.

Here is a guide.

Here is another one

2015 IRC provisions:

Thanks for the help!

Sounds as if you will need a building permit, so it will be a good idea to talk to the local building official and bounce these ideas off him/her. They will be familiar with these various options and should be able to give you a sense of which options may be acceptable and which may not. I would expect them to require you to have engineered drawings which show appropriate plans and details of construction to ensure the proper stability of the project. Good luck with your project.

Try not to point out to the inspector that the existing foundation is "stable". They may not have a good sense of humor.

In our area (coastal VA), agricultural buildings are not subject to the same permit and inspection requirements as residential construction. If it's similar in your area, the footings for the stable may not comply with requirements for residential construction. Plus, Colorado has lots of areas with expansive soils.

You will be well served to consult a geotechnical engineer (sorry, I know use of the term "engineer" is not popular on houzz.com) in your area who can design footings based on the proposed structure and the properties of the soil at your site.

Before talking to the building inspector you should review the requirements of the residential building code and the local supplement since the building inspector isn't going to design the house for you.

Here are the codes.

As mentioned above, foundations are a critical issue in Colorado so pay close attention to the Foundation Compliance Form at the end of the Residential Supplement.

The Quick Code Check feature is very helpful.

When you think you understand the basics and have investigated the existing conditions, meet with the building inspector and go over what will be required.

Here is the online version of the 2015 IRC.

If you are serious about building the house yourself, I recommend buying the soft cover version of the 2015 IRC and marking the sections that are revised by the Supplement. You can buy it from the county for $108; the ICC sells it for $134.

It's usually Cheaper to do the teardown than to retrofit.

"I know that typically you would pour 5 foot deep foundation walls (frost line for us) ..."

The frost depth for your county is set by the building code supplement and is only 3 ft.

It is unwise to make assumptions instead of reading the building code and/or seeking the advice of those who have done so.