RTFM

Annie rings the doorbell and waits. Nobody answers. She checks that she's at the right door – yes, this is grandpa Karl's place. She rings again. A window on in? one of the adjacent buildings opens and she hears Isabelle quietly calling her name.
Annie, we're over here.
Annie beings to walk over.
Which door?
Number 15, I'll let you in.

The others are inside.
Karl says: We needed a little more space. Nobody ever moved in here, so might as well use it.
They go to the living room where cables and opened boxes of computer parts are all over the floor. Lucas sits on the floor in front of the rack. The side panels had been removed, revealing the rails holding the various devices inside as well as their power cables. One of the devices, a large metal box with many boards inside, is hanging half-disassembled out the front.