This page explains the steps to upgrade from the Resident Services Tent to the Resident Services Building.
Requirements to Unlock
Invite a New Resident to Move to your Island
After you have a house, the museum, and the shop, the first step is to invite a new resident. To do that, get a Nook Miles Ticket and go to the airport, say you want to fly, and choose to use the Nook Miles Ticket. (Tom Nook gives you a free Nook Miles Ticket when you talk to him after your tent has been upgraded to a house.)
On the island that you go to with the Nook Miles Ticket, you will find an animal resident. If you want them to move to your island, talk to them twice to ask them to move in. If you don't want that animal to move in, you can visit other Nook Miles islands to try to find an animal that you want to live on your island.
Build a Bridge
After you have invited an animal resident to move to your island, Tom Nook will get a call from them the next day, during his morning broadcast. Talk to Tom Nook and ask "What should I do?" and he will give you a bridge construction kit recipe. Create the kit and build it where you want to have a bridge.
Build Housing Kits
Talk to Tom Nook again and ask "What should I do?" and he will give you three housing kits, as well as several recipes, including the ladder. Put the housing kits where you want island residents' houses to be. (You will eventually be able to move them if you want, but you can only move one house per day.)
Build Furniture for the Houses
After you have built one, Tom Nook calls you and asks you to look at the check list on the sign. For each house, you will need to build the furniture listed on the sign, and put the outdoor furniture outside of the house, and put the indoor furniture into the box next to the sign.
After you build the outdoor furniture and put it around the house, be sure to check the sign and make sure that each item has a check mark next to it, so that you know that you have placed all of the required furniture correctly.
Move Residents In
After all of the housing plots have check marks next to all items on the signs, new residents can move in. You can use Nook Miles Tickets to visit islands to find more animal residents to move in.
After all three new residents have moved in, the Resident Services Tent will close the next day, and the Resident Services Building will open the day after that.