Place a portal to a pocket dimension, where you can place other things and grow crops like a greenhouse. You can even build more buildings there.

Once Solid Foundations 1.11.3 comes out, you can even build pocket dimensions in pocket dimensions. (If you try it now you won't be able to leave the nested pocket dimension.)

Install

Install the latest version of SpaceCore and Solid Foundations..
Unzip the mod folder into Stardew Valley/Mods.
Run the game using SMAPI.

Use
Once you unlock magical building with the Wizard, go to his building construction menu. It will cost 100,000 gold, a prismatic shard, 50 void essence, and 25 iridium bars.

To build more buildings there, , use the barrel (carpenter building) or crystal (magical building) next to the exit. Do NOT build cabins or horse stables in there. (In the future they will be hidden from the menu, but for now, just don't.)

Uninstall
Make sure to delete the portal buildings anywhere in your save, then delete both the SMAPI mod and the SF content pack.

Compatibility
Compatible with Stardew Valley 1.5.5+ on Linux/macOS/Windows, both single-player and multiplayer.

See also

Dimensional Pockets