Apparently there are questions about their usefulness, I wrote a comment but then decided to make a short PSA out of it since according to my quick googling Habitats don't have a good reputation in 2.2 and it's not obvious how to use them efficiently.
A quick "just think about it for a few moments" guide:
Many people consider medical buildings being super powerful, but a fully upgraded one gives you +25% pop growth, requires 5 pops to operate and eats a strategic resource. This 25% bonus is applied to your base 3 growth per planet, and because bonuses are not multiplicative, regardless of how your species look like, that's a constant +0.75 growth points per months. An empty habitat with one dude takes 1 pop and adds +3..6 to your overall pop growth (this time it depends on your species and bonuses). Oh, and it starts with 2 pops, so just immediately send one away, and keep resettling new ones. Oh, and it doesn't have a -50% growth penalty, like freshly colonized planets. A single-pop habitat is an equivalent of 4-8 fully stuffed Cyto-Revitalization Centers. Need more? Build like 5-10 single-pop habitats. Feel free to take slow breeders on your species, it literally doesn't matter, you are outgrowing everything in the galaxy, and each habitat costs you as many Sprawl Points as an empty system, since it has no districts.
Now that I have your attention, did you notice that the Habitat Central Control is upgradable at 10 pops? Move in 9 slaves or robots (50e/move, stupidly cheap), hit the button, wait for 1 day (this upgrade is instantaneous), move them out. Now you have 5 free housing. Since pop is dirt-cheap for you now, drop something useful in that 5 housing and use a slot for something.
Hey, why stop there? Without any special traits, an absolutely vanilla serf takes 0.25 housing (you can drop it to 0.175 if you build them for it, or even lower if you got a rare special event). One Fortress (which you can, and should build in habitats) gives 3 housing. 5 pops unlock a new slot. You see where I'm going with this? Let me console up something for you super quick since I'm not using habitats in my current game for balancing reasons (they are really game-breaking). I'll be using a simple, stupid variation (1-point -10% housing perk for organics and the same for robots, but there are many great builds even for unmodified species). Sorry for the 0.9 UI scale. Note:
This habitat has 0 districts and 75 pops, costs me just 2 Sprawl Points, and gives me 86 naval capacity. I could go full fortress, but this is a mix between them and strategic resource production, just to show that they can actually be useful (see the extra 6 Motes production, despite Fortresses draining some of it). In a normal game nothing prevents you from spawning like 10 of them, and moving all your strategic resource production there.
If you don't have robots, use Chemical Bliss'd thrall slaves (0.75/0.625/0.525 housing per pop, depending on traits, compared to 0.5-0.45 of robots, but the math still works and the build is totally viable). Note that Solder's output doesn't count as production, but your Strategic Resource production does, and gets affected by Stability (up to +30%) and various pop effects. If your species are into robo-modding or gene engineering, you can get much more.
If you don't have slaves, start with an 1/5/10 pop habitat, and start dumping soldiers just before late game wars (seriously, what are you even going to do with like 2000 fleet cap in the mid game?). One fortress fits 6 of them (more with a trait), each new slot requires 5, so you can have all slots unlocked without serfs. The downside is that you'll have to do something about amenities (normally serfs produce them, note how much I have despite not having any amenities buildings and not using some of my ruler slots).
If you do have a boat-load of slaves thanks to absolutely insane pop growth, but don't want Naval Capacity, just don't use fortresses (and do a mix between upgraded and unupgraded ones). There is a '-' button near the soldier job, this way that's just free buildings.
I strongly suggest you to have something for stability (a noble, Chemical Bliss for organic solder's happiness, Syncretic Evolution, Deep Space Black Site, policies or traits, anything) before doing the fortress plan. Stick with a 1/5/10 pop variation before you are sure shit won't be exploding there. At least improve the living conditions of slaves there if you are using them (Serfs can't use Social Welfare, but Decent Conditions is likely to help). If you went for Psionics, be sure to use their special building instead of unupgraded Precinct Houses.
One last advice: manually selecting which pop to grow in exchange for a -20% penalty? Totally acceptable, go for it.
© Post "[PSA] Habitats are god-tier in 2.2 if you are using them right." for game Stellaris.
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