Direct answer
Hothouse unlocks when your greenhouse/rooftop expansion milestone completes β the same path as the unlock rooftop guide.
Prerequisites
- Complete the room / atrium expansion chain.
- Hit the rooftop greenhouse unlock requirement.
- Trigger may require the greenhouse to be built, not just unlocked.
β
Evidence note
Verified against current 1.0 expansion data; exact trigger conditions are pinned in the achievement record.
OBSERVEDSteam discussions
Community threads note greenhouse electricity bills appearing before the greenhouse is usable β logged separately in troubleshooting.
βΉ
See also My Little Garden misfires in the problem database.