Storing ladder below subfloor in kitchen
Wow, +1 for originality, but -1 for practicality. 

Usually big things (like 12’ ladders) are stored outside or in a garage. Storing it indoors will create some issues: 1) creating a solid lid that doesn’t move/warp, 2) insulation around the storage space, 3) flooring that doesn’t break when trap door flexes upon opening, 4) sealing the space when you drop the milk bottle, 

Btw, when you lean a 12’ ladder up on a wall, you’ll need a ceiling height of more than 9’ or it will be too much of a angle to lean properly around counters, etc.