There is plenty of storage with a walk-in closet in the suite bath as well as one in a minor bedroom too.

A legal ADU must have: a sleeping area of at least 77 sq ft with a closet (it can be a "studio" apartment), a bathroom (with its own sink), a cooking area (with its own sink), and then standard egress/ingress sizes. 

We sprinklered the building which would have been required anyway, but this allowed us to eliminate the rear egress staircase which eventually became the en suite bath and walk in closet in the master bedroom.

Now if it were something like punching holes in the walls or kicking the water closet loose from the mounting bolts, etc...that's something different. 

Running lines through a 70+ yr old, 6800 sq ft building & trying not disturb the tenants too much was very time consuming but I covered every nook & cranny except for the allowed exclusion of bathrooms & closets ONLY.

Many old ranch houses do not have enough closet space.

It’s a living room, dining room, eat in kitchen, bathroom with a stand up shower and then front and rear porches with an enclosed semi private small yard on the ground level, and then 3 bedrooms, a full bathroom and a pretty big walk in hallway closet on the 2nd floor.

Closet doors open and close.

Or they can drill a hole through the closet in the master bedroom so the network cable can be routed from outside and the modem hooked up in the master bedroom.The whole point of doing this is to secure a modem and router in the master bedroom since my friend will Airbnb the unoccupied rooms but doesn't want to leave the modem/router in an unsecured location for security.1) If she chooses to route the network cable from the closet, is there a good way to lay down the cable so people don't trip over it? 

We are a family of 5 so adding a couple of extra bedrooms, an extra bathroom per floor, a separate living space on second floor, extra garage space, brick/stone exterior, upgraded flooring and countertops, lots of closet space, etc.