headroom

[hed-room, -roo m] /ˈhɛdˌrum, -ˌrʊm/

noun

Nautical. the clear space between two decks.

Also called headway.

clear vertical space, as between the head and sill of a doorway, the ceiling and floor of a room, or the ceiling of a vehicular passageway and a vehicle roof, as to allow passage or comfortable occupancy: over 7.5 feet of headroom in the attic;
a covered bridge with limited headroom.
clear vertical space above one’s head, as in a vehicle or room:
plenty of headroom for passengers.

Also, head room.

Origin

1850-55; head + room

Examples from the web for headroom

The expanded attic provides ample headroom for a new sitting area.
The raising of new capital leaves some headroom for further markdowns.
But it also means that there is more headroom for expansionary policy, and more need.
Not only does it have more than enough headroom, the vestibule is covered and there is a terrific rain cover.
The design is great with ample headroom and ventilation.
Pitch the five-equilateral-triangle design tall for a smaller footprint, but more headroom.
In my world the easiest way to categorize people is by headroom needs.
My room was the attic of an old house, with generous floor space and not much headroom.
There's tremendous technical headroom for doing that.
The seats are supportive, and there's plenty of leg and headroom for all but the tallest people.

British Dictionary definitions for headroom

/ˈhɛdˌrʊm; -ˌruːm/

noun

the height of a bridge, room, etc; clearance

Word Origin and History for headroom

"space above the head," 1851, from head (n.) + room (n.).