If I wanted a backyard guest house, how deep would my pockets need to be?

Steel wool's definitely solid advice. Had a similar issue a couple years back - client insisted peppermint oil would do the trick, but mice just shrugged it off after a week or two. Ended up sealing everything tight with steel wool and some expanding foam, and that finally did the job.

As for flooring, standard subflooring usually works fine if your sealing is thorough. But I did have one project near a wooded area where we added a thin metal mesh barrier under the subflooring. Might've been overkill, but the homeowner was pretty anxious about critters getting in. So far, no complaints...and no furry tenants either.

"Ended up sealing everything tight with steel wool and some expanding foam, and that finally did the job."

Steel wool's great, but did you notice any issues with rust over time? I've heard mixed things about moisture causing it to degrade, especially near flooring or foundations...

Steel wool can definitely rust if it's exposed to moisture over time, especially near foundations or areas prone to dampness. But pairing it with expanding foam, like you did, usually helps minimize moisture intrusion significantly.

"Ended up sealing everything tight with steel wool and some expanding foam, and that finally did the job."

If you're concerned about longevity or rust issues down the road, stainless steel wool is a decent alternative. It's pricier but holds up better. Sounds like you've done a solid job overall though - nice to hear it worked out!

While stainless steel wool is definitely a solid upgrade, personally I'd skip steel wool entirely around damp foundations. Even stainless can degrade eventually under constant moisture exposure. A better long-term solution I've found is copper mesh - doesn't rust, rodents hate chewing through it, and it's surprisingly easy to work with. I sealed some gaps near my basement windows years ago with copper mesh plus foam... no rust, no critters, still holding up nicely.

"Ended up sealing everything tight with steel wool and some expanding foam, and that finally did the job."

Just something to consider if longevity matters to you.

Copper mesh is definitely a smart move for damp areas. I've wrestled with moisture issues in my historic home's basement for years, and steel wool - even stainless - just didn't hold up long-term. Copper mesh + foam combo is still going strong after 6 years... Good call.