Sally's "flat"

[Aikuzo]

But you must remember that two years ago the two main characters hadn't met, which was surprising when you consider the proximity of Andrew's cottage and Sally's flat.
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Does flat means "place" on that context? If correct, do we normally use 'flat' sometimes to represent the word "place"?
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I quit studying English years ago and have just come back to study the basics.

[Jim Jim]

Flat is an apartment. I believe that the word is derived from it being all on one floor.

I also think that it's more Brit English than American English, but most Americans would understand. (I'm American.)

[Aikuzo]

Hey thanks man!

- Flat is an apartment. I believe that the word is derived from it being all on one floor.

I'm sure these senteces are correct. I just want to play with it a little bit. Is it also gramatically correct if I say "I believe that, that word is derived from it being all one floor." there instead? Also should I had used that comma there after the first "that,"?
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I quit studying English years ago and have just come back to study the basics.

[Jim Jim]

While it's correct, most people avoid the that that construction. No comma.