Is this a focus sentence? One reason why some people believe that living in a house is better because it has?

What do you mean by a "focus sentence"? You mean a topic sentence to head a paragraph? It could be if it weren't so wasteful, incomplete, and unfocused itself. First, get rid of "is" and all the wasted words accompanying it. More words is not the same as more meaning. Then finish the comparison: better than WHAT? Then finish the sentence--are you going to focus on one item in your followup development or a list of things? So:

"Some people believe that living in a house is better [than what? an apartment?] because it has [what?]."

A focus or thesis or topic sentence or topic sentence has one primary function: it tells the reader WHAT will be developed and WHY. And the original sentence needs reworking because it does neither.

One reason why some people believe that living in a house is better is that it has more rooms than (?)

It would be better with a comparision involved?

Or if you have already used the comparison in an above paragraph then you coudl just leave it as 'One reason why some people believe that living in a house is better is because it has more rooms'