We've moved towards QR codes which direct guests to web pages or how-to videos (e.g. for a gas fireplace operation instructions) which keeps it pretty clean in the home but still gives guests what they need.

Again for examples we only have single vanity sinks in each bathroom, they had us marked down as 2 sinks, that decreased the value by $2K, they had us marked down as a whirlpool tub in the main bathroom which hasn't worked since we moved in, he removed that for $2.5K, then we have 2 fireplaces both gas but one is exhaust vented out the side of the house without a chimney which took another $1K off the value. 

We don't let tenants use fireplaces anyway but I would definitely have a cap installed. 

Maybe a really nice outdoor space with tv and fireplace?

This includes sewer, septic tank, fireplace (I had one recently that the mortar was closer to sand than cement), foundation (if there is any concern), hvac, roof (if there is any concern), permits, easements, etc.Checks are fairly cheap, repairs can be very costly.best wishes

It's about 350 sq. ft. with a kitchen, fireplace and 3/4 bath.

Out here when I live it's going to cost at least $275/SF to build a spec that will keep up with the competition and meet buyers expectations (high ceilings, semi-custom cabinet package, fireplace, floor to ceiling tile in all showers, swimming pool, temperature controlled garage, pot filler, thermador appliances, level 5 drywall, oversized custom pivot door for entry, epoxy in garage, custom closet in master, etc)Also that number assumes you are owner-builder or you GC the job yourself.

My townhouse is all-electric with no garage, no fireplace, and no bbq grill, so no source of carbon monoxide.

We can't say what it will cost you, because it varies widely because of the size of your house, the location, whether you have a fireplace or a pool or something else that would increase risk, and many other factors. 

Here are a few items to look out for:1) HVAC Filters 2) Pest control, especially here in the South3) Gutters/water4) Yard/lawn5) Fireplace/Chimney 6) Water heater maintenance 7) Dryer vent cleaning8) Door and window seals