To provide a building unit capable of decreasing grids having low frequency of use so as to restrain wrong selection of grids, and preventing an increase in type of a building unit, and a design method for the unit.

A third grid pattern S3 formed by plural short grids Gs forming a fist interval R1 and a second interval R2 different in interval size is set in the interior of this building unit 10, and it is possible to set the other grid patterns S1, S2 different in position in the direction of a short side of the second interval R2 from the third grid pattern S3. Thus, one of the grid patterns S1-S3 is selected according to the position of installing a fixture, whereby even if the number of short side grids Gs is small, the position of the fixture can be determined so as to prevent any increase in type of the building unit 10.