Digital architecture / author, Dimitris Kottas. (OCLC #872755012)

RDA 3.4.5.18 on individually paged volumes (as opposed to a continuous pagination over multiple volumes) says to record the number of volumes (units) but omit the pagination (subunits). For this set, extent would be:

2 volumes

There is an optional addition which allows specifying the pagination:

2 volumes (277, 263 pages)

The LC-PCC PS says in general not to apply the addition, but the NLA PS says to generally apply it. The copy for this set in OCLC is from National Library of Australia (Symbol AU@) which problably explain why it has been included in the copy!