Learn English – Best word for “unable to change”

I'm looking for a word that is the opposite of "adaptable." I would like to say "unadaptable," but that's not a real word according to my dictionary. So, what's the best word out there for not-able-to-change?

Update: I'm embarrassed to admit I was accidentally searching the Mac's built-in thesaurus, not dictionary, when I checked to see if unadaptable was a word. My bad! I'll still award Jasper the answer for pointing out my obvious oversight.

Update 2:

To respond to the various calls for clarification, I'm specifically using this word in a report talking about the development of certain kinds of subdivisions, especially very homogenous single-family subdivisions in outer ring suburban areas. In my report I'm referring to a neighborhood that can change at the micro-level, ie. it can improve or deteriorate, but it cannot change what it is: ie. the infrastructure pattern is not capable of supporting a different land use, and it is not economically feasible to retrofit the infrastructure, therefore the nature of the subdivision cannot be changed, while the condition of the subdivision (if it is deteriorating) may cause people to wish that it could be changed.

So, with that in mind:

1. Why not immutable?" Immutable is a great word, but I understand it's connotation to mean "permanent, unchanging", in the sense that something simply is not subject to change, not that it might like to change but cannot.

2. Why not "inflexible?" Inflexible connotes being "unwilling" to change, or perhaps being able to be changed (or reshaped / bent) but only slowly and with much resistance.

So, in this situation I am looking for an adjective that describes an inability to adapt to changing market conditions by shifting from one land use to another. In this case I specifically use the word "adaptable" in many places to describe the opposite, and given the context I think "unadaptable" will be better understood than any of the other suggestions, even though I am aware of (and agree with) the comments about it's status as a rarely used and therefore imperfect choice.

Thank you all for the feedback!