It is a small, one room house that is boarded up and inaccessible when the Lone Wanderer first arrives at Arefu. The animal holding pen will be occupied by dead brahmin due to The Family's recent raid, which included the slaughter of the brahmin and terrorizing of the residents.
The house will be referred to as Alan's residence if the Lone Wanderer has The Family agree to protect Arefu in exchange for blood packs at the end of the quest Blood Ties, as Alan will begin living there to hold up The Family's end of the deal. New brahmin will also appear outside the house once the problems with The Family have been resolved.
A mirelurk may spawn by the brahmin pen next to the entrance of the abandoned house. It will proceed to kill all of the brahmin inside of the pen unless the mirelurk is killed first.