Please try some peas.
-- Mmmm. These peas are delicious! Do you have a secret recipe?
I'll let you in on the secret: my grandma's recipe calls for a generous dash of arsenic.
-- They're scrumptious. I've already eaten the first helping you have given me.
Well then, have a second helping. But I doubt you will ask for a third.
Why do we say "the first helping" but "a second helping"?
The first helping is a particular helping: it actually existed, and was of a certain size and taste and color and temperature, and so forth, and it was served to you at a particular time.
A second helping, on the other hand, is only a possibility, a concept, an abstraction: "a-helping-that-would-come-after-a-first-helping". Its only attributes or properties are its ordinal and its category.
In the same way, "a second season" means only "a season that would follow upon the first season". Until it acquires some specific properties in addition to its ordinal (2nd) and its category (season), such as plots and stories, new characters or reappearing characters, a number of episodes, whatever, we cannot speak of "the second season".
Short answer
It's assumed that the five-room house already has a kitchen and a bathroom, and the reader is reasonably expected to make that assumption. Therefore, "the" is appropriate.
Also, "the kitchen and bathroom" can be written as "the kitchen and the bathroom", but the latter is too wordy and thus omitted for conciseness. In "the kitchen and (the) bathroom", the second "the" is understood.
Explanation
The rule for "the" isn't necessarily for its noun to have already been mentioned beforehand. Rather, if the reader (listener) can be expected to reasonably answer the question, "which one?", then "the" is appropriate.
In the case of the sentence you posted, the writer says "including the kitchen and bathroom". The reader can ask "which kitchen and bathroom?", and the reader should be reasonably expected to answer it: "the kitchen and bathroom (in the five-room house)".
Best Answer
Chalk it up to idiomatic English. It's curious how we would generally say:
but:
The same goes for the phrase away from:
The same holds true for after we get back:
When it comes to the word town, sometimes an article is optional:
Both of those are acceptable, but I think you'll find the first one is more common and idiomatic.
Interestingly enough, I would never omit the article with the word city: