You're stuck in a tiny tube. It's cramped. You’ve got a fabricator on one wall, a locker full of titanium on the other, and a single bed squeezed into the corner. It's basically a studio apartment underwater, and quite honestly, it sucks. If you want to survive 4546B without getting claustrophobia, you need to go vertical. Figuring out how to make second floor in Subnautica isn't just about aesthetics; it’s about base integrity and actual sanity.
The game doesn't give you a "build floor" button. That would be too easy. Instead, you're playing a weird game of underwater Tetris where the physics sometimes decide to stop working because a piece of kelp is in the way.
The Vertical Connector is Your Best Friend
Forget everything else for a second. If you want a second story, you need the Vertical Connector. It’s a simple tube. It looks like a Pringles can made of reinforced glass and titanium. You can find the blueprint for this pretty much immediately, as it’s part of the basic base building set.
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Here is the thing people mess up: you can't just slap a room on top of another room and hope they merge. It doesn't work like that. You build your bottom Multipurpose Room first. Then, you swim outside, aim your Habitat Builder at the roof, and place a second Multipurpose Room directly on top. It will snap into place. You'll see the green outline. Click. Done.
But wait. You’re still stuck in the bottom room. There's no hole in the ceiling. This is where the ladder comes in.
You have to go back inside. Bring titanium. Equip that Habitat Builder again and point it at the center of the ceiling. If you’ve aligned the rooms correctly, the game will let you place a ladder. Suddenly, a hatch appears, and you have a two-story villa. It’s that simple, yet so many players spend twenty minutes swimming around their base trying to "stack" I-compartments, which is a total nightmare for pathfinding.
Multipurpose Rooms vs. Large Rooms
If you're playing the original Subnautica, the Multipurpose Room is your bread and butter. You find the fragments for this at the Jellyshroom Cave Abandoned Base or the Floating Island. Seriously, go to the island. It’s safer. You don’t have to deal with Crabsnakes trying to eat your face while you’re scanning blueprints.
Once you have the Multipurpose Room, stacking them is the standard way to grow.
Now, if you’re playing Subnautica: Below Zero (or the 2.0 "Living Large" update for the original game), you have the Large Room. This thing is massive. It’s beautiful. It also makes building a second floor feel different because of the internal partitions. In a Large Room, you can actually place "Large Room Glass Domes" on the top floor, which gives you a stunning view of the Reefbacks or the Stalkers trying to chew on your camera drones.
Why Base Integrity Actually Matters Now
Every time you add a second floor, you’re adding weight. Subnautica calculates "Hull Integrity." If that number drops below zero, your base starts leaking. You’ll hear that terrifying alarm, and water will start spraying through the seams.
- Reinforcements: Stick these on the bottom floor walls. They add +7 integrity.
- Foundations: These give you a flat surface to build on and add a bit of strength.
- Bulkheads: These are doors. Use them. If one floor floods, a closed bulkhead keeps the other floor dry.
Honestly, I usually dedicate my entire "ground floor" to heavy machinery and reinforcements. I cover the walls in lead-lined plating so the second floor can be all windows and glass. There is nothing better than waking up in your second-floor bedroom and seeing a Ghost Leviathan in the distance through a floor-to-ceiling reinforced window.
The Moonpool Conflict
The Moonpool is a weird beast. You can't really "stack" a Moonpool on top of another Moonpool in a way that makes sense, because your Seamoth or Prawn Suit needs to enter from the bottom. However, you can build a second floor of living space directly adjacent to the top of a Moonpool.
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Use an I-Compartment and a Vertical Connector to bridge the gap.
A lot of players try to build their entire base at one level. Don't do that. Use the verticality of the ocean. Build a "basement" for your bioreactor and water filtration systems. Use the middle floor for storage and crafting. Make the top floor your observation deck. By spreading your base across three or four floors, you actually reduce the "footprint" on the seabed, which means less clearing of rocks and flora.
Common Glitches When Stacking
Sometimes, the game says "Target position occupied." This is the bane of my existence. Usually, it's because:
- There is a locker or a wall planter too close to the ceiling in the room below.
- A Fish is swimming exactly where the new room should be (yes, really).
- You haven't fully built the room below yet.
Make sure the bottom room is 100% finished. No blue ghost outlines. If you left a single titanium out of a reinforcement panel, the game might treat the room as "incomplete" and won't let you stack anything on top of it.
Making it Look Good
A vertical base looks like a skyscraper if you do it right. If you’re feeling fancy, don't just use ladders. Use the Alien Containment unit. If you stack Alien Containment units in Multipurpose Rooms that are on top of each other, they merge into one giant, multi-story aquarium. You can swim from the bottom floor to the top floor inside the fish tank. It’s easily the coolest way to handle how to make second floor in Subnautica while also showing off your Cuddlefish.
Think about lighting, too. Bases get dark. If you have a second floor, the light from the surface might reach it better if you're in shallow water. In the Deep Grand Reef? You're going to need a lot of internal lights or some Creepvine Seed Clusters in a pot to provide that natural bioluminescent glow.
Actionable Build Plan
Stop overthinking the layout. Here is the most efficient way to get it done right now:
First, build a Foundation on a flat area of the Safe Shallows or the Grassy Plateaus. Place your first Multipurpose Room. Head inside and build a Bio-Reactor or a few lockers, but keep the center of the room clear. Go back outside, climb onto the roof of that room, and snap a second Multipurpose Room on top.
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Once that's built, go back to the bottom floor. Use your Habitat Builder to place a Ladder in one of the two available spots (usually center or slightly off-center). Now, climb up. You’ve successfully doubled your square footage.
From here, focus on Hull Integrity. If you start adding glass corridors or Windows to that second floor, immediately go back to the first floor and add a Reinforcement panel to a wall. Keeping the "ugly" reinforcements on the bottom floor keeps your top-floor views pristine. Finally, consider adding a Solar Panel to the very top roof. Since it’s higher up in the water column, it’ll catch more sunlight and generate power more efficiently than if it were tucked away on the sea floor.
Get your titanium ready and start building upward. The ocean is deep; your base should be tall.