You are quite right. The adjective does come before the noun. The two sentences are different grammatically because they have different meanings.
alternative condominium = adjective + noun
condominium alternative = noun + noun
In the phrase 'an alternative condominium' , the word 'alternative' is the adjective, and 'condominium' is the noun.
In the phrase 'a condominium alternative', the word 'alternative' is the noun. The word 'condominium' gives us more information about what kind of alternative it is. When a person is deciding where to live, they have various options - for example, they might have a 'house alternative' or a 'condominium alternative'.
Noun+noun compounds are very common in English. The second noun tells you what the thing actually is, and the first gives you more information about it. For example, 'a winter jacket' is a jacket which you wear in winter.