For a given layout the locations next to blocks are unusable. In the layout above there are 3 wasted spaces. In another layout there is one wasted space less.
wasted (adj.)
Unprofitably used, made, or expended wasted effort
M-W
Not exploited or taken advantage of
a wasted opportunityUseless or unprofitable
wasted effort
Collins
Crucially, you will start to see little islands of space where there are no pen lines whatsoever. These areas are usually cut off by an intersecting route and therefore rendered essentially unusable. Such dead, wasted spaces are critical in adding usable floor area, especially in flats or apartment blocks where physically adding square footage by extending in not an options.
Laura Clark; The Handbook of Home Design (2023)
Where there is utility, it is invented and springs forth after the fact, as in the case of the spandrels of the Basilica di San Marco, which ended up being used, much later, as mere decorations. Originally drab and useless, these wasted spaces now have the accidental function of letting the golden light of Byzantine mosaics shine brighter.
Barbara Stiegler; "1880: First Philosophical Critique of Adaptationism" in Martin Hartmann and Arvi Särkelä (eds.); Naturalism and Social Philosophy (2023)
Unavailable often implies that something is currently occupied, is being used for another purpose, or has been designated for future use. Wasted usually means serving no purpose or a purpose of little value.