The 5 building upgrade strategy step by step

1. Put all non-maxed buildings in storage except 5, including the one you want to work on (ie the “target” building). The other four buildings don’t matter - you will only be working on the target building.
2. If there is an upgrade on target building, go to step 3. If there is no upgrade on target building, cancel an upgrade on one of the other buildings. Go to step 5.
3. Upgrade target. A new upgrade will appear on another building.
4. Cancel upgrade on any building. Upgrade will go back to target. You can do this cancel once every 4 hours.
5. Upgrade target. A new upgrade will appear on another building.
6. Cancel upgrade on any building. You’ll have to watch an ad this time. Upgrade will go back to target. You can do this cancel once every hour.
7. This building will now have no upgrade. Put it into storage.
8. Upgrade target. Upgrade will reappear on target with timer.
9. Remove building from storage.
10. Wait for timer to run on target.
11. Complete steps 3-10 until building is fully upgraded. You will actually skip steps 3-4 until 4 hours have passed.
12. When target is fully upgraded, select new target building, bring a new building out of storage and repeat.

I will upgrade buildings other than the target building ONLY if the upgrade doesn’t require plans.

Using this strategy I’m able to max a building in 1-2 weeks depending on starting level and availability of plans. Plans (especially red) will be the limiting factor so make sure to do the ship task as often as possible - and watch the ad to double the amount you get.