What is the optimal base layout?
That's a difficult question, as most adjacency bonuses have been removed, having an optimal layout is not that important anymore. It's more a question about WHEN do you build a facility rather than WHERE and this comes down to personal preference and needs. E.g. Psi-Lab is something you'll want early, if you want to heavily use the class, but you may wait a long while, if you don't feel they are necessary. AWC is important to get, if you suffer from a lot of wounded soldiers, but doesn't do all that much in the early game otherwise. What I can say, is you want to get the GTS and the Comms quickly on higher difficulties.
The only base-placement you have to worry about is the workshop, as it still provides adjancency bonuses, regarding that, please check this question: Which buildings benefit from engineers? There may be an optimal placement strategy, if you can build 2 workshops, but that's not possible, I finished my workshop and cannot build another on the tile next to it.
I find it's best to optimize placement, instead of rushing certain things like in XCOM 1.
Workshop should be placed in the middle, second row down. A lot of the other facilities should be built around the workshop.
Your first facility should almost always be the GTS. The rookie training and extra squad size makes a big difference in early survival.
Second facility would be the power. Build it on a power conduit, even if you have to dig to the bottom. It's worth doubling up on engineers to dig that far.
Third would be either Proving Grounds or Comms.
Comms obviously gives you money. But unintuitively, you may not want to rush this. You'll want to spread into the direction of alien facilities rather than going for cash and region bonuses. The facility itself only gives 1 link; you'll have to dedicate an engineer for 2 more. So it depends very much in your engineering capabilities.
Proving Grounds tends to have unlimited projects, so you'll want to start early. The skulljack/skullmining gives you strategic bonuses that make the game much easier. Mid game, this becomes the bottleneck.
Workshop should be finished quite early as it's cheap, low power and effectively gives you bonus engineers. It can help you dig or simply assign engineers to facilities that have to be constantly manned.
Psi lab should be done early if at all. Training psionicists can take months.
AWC is more a mid game thing. Early in the game (pre armor), your best chance at survival is to keep the enemy from shooting at all. It's better to put those supplies into flashbangs, skills, and guns. Once you get decent armor, you'll have a lot more 'gravely wounded' people who are not dead. AWC becomes great here.
The laboratory is not that helpful. It barely increases research rate. Mid game, research isn't even your main bottleneck... engineering is.
Defense matrix is very rarely useful. Again, whatever resources here might as well go into better weapons and armor.
Due to the lack of adjacency bonuses, it matters far less than in the first game. Therefore, there are only really three guidelines to try to follow:
- Do not put the Workshop against the edge. It can provide a "free" engineer in any (non-diagonal) direction, so this maximises its flexibility.
- Only put facilities that can use engineers next to the workshop.
- Try to build power relays on power coils. They produce much more power this way. If you can't do that, then put your most power-intensive facilities on the power coils so that their power requirement is reduced to zero.