in the US, you rent an apartment, but you own a condominium. People may refer to a place that they own as an apartment, but they do so mostly out of ignorance. In legal terms, they own an apartment.
A condominium map is a document that provides a legal description of your unit and a layout drawing that depicts the dimensions of your unit. If you change your layout, then you must get approval from your condo association. If you change the physical dimensions of your unit, then you must file an amendment to your deed, along with an amendment to the condominium map, with the county.

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Note added at 18 hrs (2008-09-03 17:24:45 GMT)
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CORRECTION: YOU OWN A CONDOMINIUM, not an apartment.

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Note added at 18 hrs (2008-09-03 17:35:10 GMT)
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I am a real estate developer, and we build "office condominiums," which are small office spaces (1,200 sq ft). Sometimes buyers come in who only want 600 sq feet, or who want 1,800 sq feet. In order to accommodate them, we have to demise off a section of another unit and/or attach it to an existing unit. This involves amending the condominium map and updating the legal description to show the new dimensions and recording the new ownerships.

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Note added at 20 hrs (2008-09-03 19:38:41 GMT)
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ALSO: CONDOMINIUM PLAT MAP

http://www.cohoalaw.com/governance-a-condominium-map-for-dir...

http://www.estesnet.com/ComDev/Brochures/Planning/condoterms...