Grandpa's father won the Sunset Arms in a card game in 1890s, and that's where Grandpa was born. The house is very old. During the American Revolution (including the Tomato Incident) it was the local center of the resistance against the British (Hey Arnold!: The Movie).In the episode "Door Number 16" it was mentioned that 13 boarders live there, but it has only 1 bathroom — however it has been shown to have at least 2 private bathrooms: Mr. Smith's, and Grandpa's ("Four-Eyed Jack"). Ernie Potts claims that the building is made of 2873 bricks ("Fighting Families"). There are several inconsistencies with the layout of the building as well (see below). It is shown to be much bigger than it looks, and Oskar and Suzie's room is shown to have its own private kitchen. Mr. Hyunh's room is also shown to have a small kitchen in ("Gerald Comes Over"). Room 16, according to Oskar in Door Number 16, is a large split level suite equipped with its own bathroom, as well as other luxurious accessories. The building does not have central air-conditioning, as shown in the episode "Heat" when Grandpa and the boarders are seen gathered around a window air-conditioner, only to have it conk out as a result of a brownout from the heat wave. The boarding house has an old hot water heating system with radiators located throughout the building. In "Snow", the furnace/boiler fails, and Arnold has to help his Grandpa fix it.
Arnold's bedroom seems to be located in a converted attic, as it has pull-down stairs. The roof is also entirely made of windows, some of which can be opened. Arnold's room is very high-tech, with lights, a stereo system, and a hideaway couch, all of which can controlled with a single remote-control. Helga once got trapped in the space behind the couch in the episode "Helga's Parrot". She even took a close overview of the building's inner structures in the episode "Helga Blabs It All". There are also a couple of hidden storage areas with sliding doors that can be opened and closed via remote control.
The first floor of the boarding house has a nicely equipped eat-in kitchen, separate dining room, living room, and family room. The second floor has an untold number of bedrooms, and one communal bathroom. The unfinished basement is largely used for storage, and is home to the furnace, but also has a laundry room with coin-operated washers and dryers, and Grandpa's private bathroom that he managed to keep a secret from everybody, including Arnold and his own wife, until the episode "Four-Eyed Jack".
The boarding house also has a large rear-entry garage, where Grandpa parks his beloved green Packard. Arnold and his friends build a float atop an old car in this garage in "Runaway Float". The boarding house has no front yard, as the front doors exits right out onto a public sidewalk, but it has a decent-sized backyard. In many episodes, the boarding house is shown to have a purple 1940s Chevrolet coupe sitting on cement blocks in the backyard.
The roof of the boarding house can be easily accessed from inside the house via a staircase, and it is shown to have a water tower (The building dates back to the 18th or 19th century but it probably has modern public water now). Arnold's Grandma also had her new grand piano lifted onto the roof when the movers could not fit it through any of the doors or windows in the episode "The List".
Only four boarders (other than Arnold and his grandparents, who live in Room #1) seem to be permanent residents:
- Oskar and Suzie Kokoshkas: Room #5 ("Arnold As Cupid"); Room #9 ("Gerald Comes Over"); Room #7 ("Baby Oskar", "Oskar Gets a Job")
- Ernie Potts: Room #7 ("Arnold As Cupid", "Gerald Comes Over"); Room #5 ("Oskar Gets a Job")
- Mr. Hyunh: Room #3 ("Arnold As Cupid", "Oskar Gets a Job", "Mr. Hyunh Goes Country"); Room #21 ("Gerald Comes Over").
- Mr. Smith (Room #16 ("Door Number 16"); Room #6 ("Gerald Comes Over"))
- Lana Vail (Room #6 ("Abner Come Home","Gerald Comes Over", "Heat")
- Mr. Purdy (Room #6 ("Gerald Comes Over")
- Rhonda, and her parents Buckley and Brooke ("Rhonda Goes Broke")
- Gerald (Room #5) ("Gerald Moves Out")
- Dino Spumoni and Don Reynolds ("Dino Checks Out")
- Baby Oskar ("Baby Oskar")
- Even though Hillwood is confirmed to be in Washington State (as said by creator Craig Bartlett), it served as a decisive center for battles against the British in the American revolution. This is impossible as main settlement of Washington began around the 1830's, long after the Revolution ended.
The layout of the boarding house is extremely inconsistent. Aside from the obvious inconsistency with which rooms numbers belong to each border, there are several other inconsistencies worth noting.
- The number of rooms in the boarding house seems to fluctuate throughout. In the episode "Gerald Comes Over", there seem to be enough rooms to house Grandpa and Grandma, Ernie, Mr, Hyunh, Oskar and Susie, Mr. Smith, Mr. Purdy and Lana Vail, making a total of at least seven rooms in the boarding house. However, as the show progressed, the layout became more realistic. The Kokoshka's, Ernie and Mr. Hyunh became permanent residents. If one is to count all of their rooms, as well as Dino Spumonis room and the spare bedroom used by Gerald in "Gerald Moves Out", the number matches the original number of rooms. Therefore, it can be concluded that the boarding house most likely has six bedrooms/apartments.
- In some episodes, there is an archway separating the hallway between the bathroom and Grandpa and Grandmas bedroom and the rest of the borders rooms. In others, there is none whatsoever.
- The placement of the basement stairs has varied from the episode "Four Eyed Jack" to "On the Lam". IN "Four-Eyed-Jack", the basement is accessed through a stairwell behind the coats in an area under the main house stairs. In "On The Lam", the basement stairs are accessed via a door in the family room of the house. It should be noted that there are possibly two basement stairs in the bording house. The "main" stairs are most likely the stairs as they were in "On the Lam" These stairs would lead to the unfinished basement with the furnace and the washing machines. The "secret" staircase is the one under the house stairs, leading to Grandpa's secret bathroom. This is affirmed in Four-Eyed -Jack; all of the borders, including Arnold, and excluding Grandpa, had no idea of the existence of the extra bathroom. Phil could not have hidden it if the secret stairs were the only staircase in the house leading to the basement.