In this Phase, the players come across this plot of land somehow. They could either simply claim it as their own after stumbling across it, or perhaps they were sent here on someones behalf. The party may have even come into possession of the deed for the land, really whatever works best for your group and game.
These are meant to be used over time as the base is slowly built by locals / contractors while the party is away on missions. However, there's nothing stopping the party from hunkering down and really putting their backs into it. In this case, they could clear multiple stages in a day or two.
Upon first arriving in 'Stage 1, Phase 1, Begin', you might consider having a few combat encounters to clear out the surrounding area of threats before the players set to work. If they decide not to check the area, this would be a great time to ambush them as they set up camp for the night. This base is designed to grow with a low level party in mind, though it can technically work at any level.
The Build-A-Base map was created by ZeroG for BBEB. Check out the rest of our maps at https://patreon.com/BBEB