Free meaning “free of charge” vs “unoccupied”

Solution 1:

The following are free from ambiguity:

Is this seat taken?

How much does this chair cost?

It's unlikely that you need to be so precise, though, since there's usually enough context to distinguish.

"There is a free room in the house."

is ambiguous, but I would assume it meant "an available room".

Solution 2:

"Is this chair free?" would not ordinarily be misunderstood, since presumably you and your interlocutor would have the empty chair before you (or beside you).

Another common US idiom, which avoids the problem, is "Is this chair taken?"

If on the other hand you were asking where you might find a place to sit, you would probably ask "Is there a chair free anywhere?"

In a commercial context "a free room" would indeed imply that no charge is made for the room. This implication may be avoided by postpositing the adjective; you might be told: "There's a room free in the house".

Again, however, a different word would be more likely: "There's a room available".

In a non-commercial context, where there's no ambiguity, free is acceptable: "If you need a place to crash we've got a room free."

And if you're asking for a room either would serve: "Do you have a room free?" or "Do you have a room available?"

Solution 3:

The English word free has several meanings, some of which can be easily conflated:

• Unoccupied: The room is free. I heard this a lot in college, when we had to schedule the rooms for meetings/classes. A "free" room was one that had not yet been scheduled, and so was vacant.
• Costs Nothing: Here's some free advice. This is the most common connotation of free, and sometimes people take advantage of that to conflate ideas.
• Unrestricted: Free speech. This tends to be used a lot when discussing what one may or may not do. This is what people mean when they say "it's a free country".
• To set free: as in "Free all the prisoners and we'll let your wife go".

There may be others. If anyone else comes up with one, please add it.

Solution 4:

Yes, in English "free" can mean "no charge" or it can mean "available". So, "Is this chair free?" could mean "Can I use this chair without paying?" or "Is this chair not being used by someone else?"

It's usually clear from context which you mean. (I'm sure there are cases where this ambiguity has caused embarassment, like someone asks if something is free meaning no charge, the other person thinks he is asking if it is available for purchase, and so the first person begins using it or carries it off and then is surprised when he is asked to pay.) When in doubt, use different words. Like ask, "Is there a charge to use this chair?" or "Is this chair available?"