As the name of the category says, the goal of this run is to meet all the unlock criteria for every building in the game (with the exception of Import Wear; see the end of the guide for the reason why it is not required). The basic requirements for the run are having at least 10 Miis (at least 3 male and 3 female), solving 50 problems on the island, having 5 treasurers, and having two Miis marry and have a child that grows up to leave the island.
Set the date and time to be in the middle of the day so that no Miis will be sleeping, increasing the number of Miis that could have problems. Upon setting the date and time, create your first Mii, giving them a one-character name and making sure that the Mii (as well as all other Miis you create over the course of the run) is considered to be a grown-up. Upon going to the food mart, buy the max amount of veggie burgers - this is the cheapest food present in the first slot of the food menu when you give it to the Miis, and giving the Miis food is one of the most common problems in the game.
After creating the second Mii and unlocking the Town Hall, talk to your first Mii again, pressing X or Y while in the apartment to unlock the Plane and the Fountain together. After the news flash ends, go into the Town Hall and make at least 10 Miis, making sure that at least 3 are male and 3 are female, and all of them are grown-ups. My personal recommendation is to have an even number of males and females, as this maximizes your odds of finding sweethearts later on. While 10 Miis is the minimum needed to complete this run, I typically create 12-14, as I find that the time lost from creating the extra Miis is made up for with the increased problems available per reset, and the increased sweetheart chances.
After creating all of the Miis, go to the apartments and start solving problems (and playing games, as you need 5 treasures to unlock the pawn shop). Once a Mii hits level 2, give them a song to unlock the Concert Hall - give all other Miis items, as they are the quickest option, and also unlock the Observation Tower. If there are no Miis with problems, save the game and time travel into the future to regenerate the problems. Prioritize Miis with hearts - having a pair of sweethearts will unlock several buildings. My general strategy with sweethearts is to try to get as many sweethearts as possible until one pair gets married, in which case I will ignore the hearts of all the non-married Miis.
Once you solve 50 problems, you should unlock the Photo Studio. Next begins the RNG-heavy part of the run - you should almost definitely have at least one pair of sweethearts, if not a married couple, by this point in the run. Upon getting the Photo Studio and the Pawn Shop, your sole remaining goal is to have a married couple and have them create a baby. This is done by time travelling and resetting until you have a marriage, and then until the married couple wants to have a baby, and then until the married couple has a baby, and then until the baby is fully grown up. This will likely take a lot of resets and be as long, if not longer, than getting the 50 problems!!!
After the baby is fully grown, make sure that you select for it to leave the island, and enable StreetPass to unlock the Dock and the Campground. Timing for the run ends upon the fading to orange of the news flash upon the creation of the final building. Congratulations on unlocking (almost) every building in the game as fast as possible!
(Note: While it is not required in the speedrun, if you would also like to unlock the final building, Import Wear, you have to also enable SpotPass and go into the town hall, select options, StreetPass/SpotPass, and Check Now. Then, upon saving and resetting the game, Import Wear should be unlocked. The reason this is not included in the main speedrun is that unlocking Import Wear requires both an internet connection, and for Nintendo to keep the SpotPass servers alive, which is unknown how much longer they will be kept up.)
