Add a new subrule to the rule “Farmland” if it exists.  Call it “Storage Space” and give it the following text:

Each Plot may have up to one Shed on it.  A shed may be small or large; a small shed is represented by a lowercase ‘s’ and a large shed is represented by an upper-case ‘S’.  A Farmer whose plot does not contain a Shed may spend 50 Sun to construct a small shed in any empty Square on their Plot.

Sheds have large shadows; the Shade of a square orthogonally (not diagonally) adjacent to a shed is considered to be 1 higher for the purpose of every rule except the one named “Farmland” and this one.