Determine the area of square.

The side of octagon is 8.
Determine the base length or height of a triangle.

So height and base of triangle is equal to h = 6 and b = 6 respectively.
Determine the area of 4 triangle.

Determine the area of octagon by substract area of 4 triangles from area of square.

Thus area of octahedral room is 328 square feet.