Building a Guild

Crab Island, home of Torchlight
in Anminster. Art by S.Armstrong

It's fun to start a club! It's even funner to build your base! There's plenty of official and homebrew stuff out there for downtime activities and guild building. In our campaign, the guild Torchlight works for the people of the Western Heartlands that can't get help anywhere else. Rejected tieflings, goblins, orcs who have jobs to be done that the city or town guard refuse to investigate come to Torchlight. Torchlight also devotes a majority of their time to investigating any strange events, or paranormal ones. Using their skills to cleanse the world of Toril of haunts, ghosts, and monsters, their reputation is becoming bigger and bigger!
One homebrew supplement I read in my search for guildish things was The Guildhall, by David Dias. Inside there's pricing for building the guild, construction costs, days to build it, and the types of rooms that would be necessary for the money put into it. As well, it goes over the maintenance costs and the kinds of workers that would be hired to tend to the guild, both skilled and untrained. In addition, if you wish to build add-ons at a later date there's guildhall upgrades, where you can add things like a jail, smithy, tavern, stable, library or laboratory and the benefits that come with the addition (for example a stable gives you lodgings for your mounts for free). There's also a section in the Dungeon Master's Guide on pg. 128 on building your stronghold, as well as Fortresses, Temples & Strongholds, from Warlock Homebrew.
It's also good to sketch out what you want your guild to look like, with a map, room locations, and so forth, bringing your guild to life for both you and the players.

Torchlight Map. Art by S.Armstrong

There are many things you, as a Dungeon Master, can do once the guild itself is established. Some ideas would be, an alliance or rivalry with another guild. There could be an attack on the guild. They could hold their lofty parties and let the social intrigue commence! Build on new characters for your MDK (Murder, Death, Kill) campaign, so the NPCs that join the guild as members can replace your fallen heroes down the line! Secret rooms, and weapon stashes - basically the lair of the heroes, gives them a foothold in the town/city they build in, making them part of that town as far as money coming in, and helping to guard and make larger decisions for the townsfolk which they protect.

Perhaps after a great battle and defence of the town they are established in, the town mayor invites them to have a seat on the current decision making circle and they can start influencing the direction of the town itself.
Whatever your reason for building a guild, or for the players to want one, it gives your players a place to call home, to refresh, to regroup and fulfill their downtime activities or... of course, build a secret sorcerer's lair in the basement.