Well your choices are really to either put more subpanels (moving some circuits from the existing panel to the subpanel to make room for the feeder breaker) or replace one of the existing ones with more spaces available.

You may wish to consult an electrician, especially to check over the existing harry homeowner work. By the way, there's nothing wrong with a subpanel not having a main breaker inside it (provided it is in the same structure). The feeder breaker in the panel it is fed from protects the wiring/panel. This is why they make main lug style panels to begin with.

You really need to have a pro look at this. You don't say. but I guess you probably have 30+ circuits full. We have a very large home with a pool that is handled with a 200amp panel with about six unused places in our panel.

Thanks for the responses. I agree, I can't do this on my own. Just wanted to get an idea of what the solutions could be to see if any money could be saved. We live in an area where work generally costs on the high side, if not above...

I'm thinking that swapping the 100A sub panel to 200A is probably a $3-4k all in?

If it weren't for your mention of a 40-space panel in the garage, I'd think you just moved into a home my friend just moved out of (we only put a 12-space in the garage, so I know it's not)

How many spaces total in the two house panels? When I first read this, I immediately pictured some really big ones, and was going to ask if you had a seperate circuit breaker for every receptacle in the house!

It sounds like you have a LOT of circuits... and need to add more!

I have two 40 space sub panels that are largely full in my house and the main panel has a dozen circuits for the hangar/garage in addition to the feeder breakers.

The worst one I've done so far had a full 40 main, 24 on the other end of the house, with two 8's on another level... That was following my OCD: seperate circuits for every fixed major appliance, dedicated lighting circuits (I normally get by with 1 or 2, but this house had a lot of lighting, and totalled 8), and a minimum of one receptacle circuit per room... I guess that's 80 total, so I suppose I withdraw my original statement. :)

Are you sure all the breakers are in use? Maybe somebody punched all the tabs from the panel cover and just put in unused breakers to make it safe. Your next step should be to map all circuits; maybe some can be combined.

If it was DIY, they may have purchased used panels that were loaded with breakers - it's worth checking as hrajotte says.