Expanding a store bought chicken coop.
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Hard to tell in that pic but did the coop expand or just the run?
that coops back wall taken down could be made however long you want it with run under the coop.
How many birds are you expanding for? or how many do you plan for?
id say your looking for 20-24sq ft in your coop. 50-60 in the run and 3 nesting boxes should be ok. Jugging from that picture it may need to grow twice its size. But that's only a guess with no dimensions to go on.
Also depending on your locating could mean the birds may or may not be confined for weather. Do the birds have access to free ranging at any time?
Pull out the nesting boxes and move them down below the 'floor' of the coop. Then pull out the floor. Remove the ramp and the wall that divides the coop from the run. Cover up the hardware cloth run parts with plywood. Remember to leave some hardware cloth exposed at the peak of the roof for ventilation. Install some roosts. Build a new run. You now have a coop large enough for about 4 to 5, depending on how large the birds will be as adults.