You have all kinds of options. Most of them will probably work. I put a second coop on the far end of my run and put a fence across the run so I can isolate part of the run with the second coop if I want to or just let them have access to everything. That gives me a lot of flexibility in managing younger chickens and in integration issues, but it likely won’t help you reach your goal. I find the young ones eventually want to roost in the main coop with the big chickens when they mature.
The way I’d set it up would be to build the new coop next to the old coop. You could tear a wall out and just make it bigger, but I would not do that. Two coops give you so much extra flexibility in dealing with things. Make a connection, maybe some type of tunnel say a foot wide and 18” high, so they can get from one to the other but set up a way you can block it if you wish and lock them in either coop.
I’d set up enough roosts in one of the coops to handle all the chickens you expect to have. I do think they’ll want to all roost together eventually. Have enough nests in that coop for all the hens too. It might get crowded.
In the other one I’d have some roosts and a nest. Any new pullets will probably want to lay where the old ladies are laying but I’d put a nest anyway. I’d also section the run (or build a new run for the new coop) so each coop can have a run but the whole thing can be opened up to form one big unit. A pop door in the fence between the runs would work great. They’ll need to go from coop to coop and from run to run.
The reason you need extra space for more chickens is that chickens have developed methods to live together in flocks. One of those methods is for the weaker to run away from the stronger when there is a confrontation. With two coops connected like this, the weaker can escape to a totally separate coop. Or they can hang together if they choose.
Some possible uses to two separate coops with a run:
1. Really help with integration issues.
2. Provides space for the weaker to run away or avoid to start with. You can expand your flock.
3. Possible isolate-a-broody location, either hatching or raising chicks.
4. Isolate an injured chicken while it heals.
5. Possible bachelor pad.
6. Grow-out coop and run.