The hardest measurement in a kitchen. Two walls meet at an angle that is rarely ninety degrees, two cabinet runs converge, and the doors have to open without hitting each other.
Almost none are, and the amount matters because a corner cabinet is a rigid box that assumes ninety degrees.
Measure 3 feet along one wall from the corner and mark it. Measure 4 feet along the other wall and mark that. Now measure between the two marks. If the corner is square that diagonal is exactly 5 feet.
Anything more than about a quarter inch out and the cabinet will not sit flush against both walls at once. It will meet one and gap at the other, and the gap has to be absorbed by scribing or by a filler strip.
Elsewhere, a run of cabinets absorbs error gradually across several units. In a corner, the error concentrates at a single point where two rigid boxes meet, and it shows as a visible wedge-shaped gap.
Along each wall from the corner, the distance to the next fixed obstruction: a window, a door casing, an appliance, the end of the run. This tells you how much space each leg of the corner has.
Wall depth available, which is the standard cabinet depth unless something is in the way.
Anything in the corner itself. Pipework, a waste stack, a radiator pipe rising, or an electrical box. The corner is where services frequently run vertically, and a corner cabinet is exactly the unit with least room to accommodate them.
Skirting and pipe boxing, which stop the cabinet sitting fully into the corner. Measure the projection of anything on the walls at floor level.
A corner unit has a door, and so does the cabinet on the wall beside it. Those two doors are close together and they swing toward each other.
Measure the intended door width of both the corner unit and its neighbour, and check that when both are open they do not collide, and that neither is blocked by a handle on the other. This is a real and common problem on blind corner units, where the door and the adjacent drawer front end up fighting.
Handles make it worse. A protruding bar handle on one door can prevent the neighbouring door opening at all, and that is not visible on a plan drawing.
A blind corner unit runs into the corner with part of its width hidden behind the adjacent cabinet. You need the total wall run and the width of the neighbouring unit, because the blind portion is the difference.
A diagonal or angled corner unit cuts across the corner at 45 degrees with a single door facing into the room. You need the distance along each wall that the unit occupies, and this is where an out-of-square corner causes the most trouble because the unit is designed around an exact angle.
A carousel or lazy susan unit has rotating shelves and needs internal clearance to spin. Measure the depth available carefully, because the mechanism needs its full radius and cannot be trimmed.
Same problems higher up, plus one more.
Measure the corner squareness at wall cabinet height as well as at floor level, because walls lean and the two are often different. A corner that is square at the floor and out at 54 inches will not take a corner wall unit cleanly.
Also check what the corner unit's door does relative to the wall it sits against. On a tight corner the door can foul the return wall before it is fully open.
In a kitchen layout, corners are the fixed points and the runs between them absorb the adjustment. Establishing exactly what the corner will take, and how much wall each leg consumes, gives you the space remaining for everything else.
Designing the runs first and fitting a corner into what is left is how kitchens end up with a filler strip wider than a drawer. The full kitchen measuring method is here, and countertops over a corner have their own considerations.
If none of the above is quite right, the eight we recommend most often are on our front page.