Josef wrote:Upstairs, downstairs are the only forms.
However, they are singular, not plural:
The upstairs is cleaner than the downstairs.
Headquarters (as a noun) is the only form. It can be treated as plural or singular:
The company's headquarters are in Bristol.
The company's headquarters is in Bristol.
The purist would probably opt for plural.
Josef wrote:The adjective is usually "downstairs" but "downstair" is possible:
a downstairs room
a downstair room
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