Without knowing your level, it's hard to answer this one.
If you're not max level yet (55 at the moment), then gaining levels will increase your max housing.
If you can build a Permits Office, that will increase your max housing. Upgrading it will improve the boost, but for upgrades 4 and up it costs nanopods.
You may need to use more efficient buildings. For example, small warehouses are incredibly worker inefficient. Replacing them with better warehouses can get you the same benefit for far fewer workers. This is one area where spending nanopods can REALLY pay off.
And sometimes, you just have to shut some of your buildings off. There's a small green switch in the upper left or right corner of most buildings' menus; tapping it will shut the building off, freeing up most of the workers. You can use this to turn off resource generators that you don't need, barracks and factories you aren't currently using, and shops that have low yield but are frequently needed for missions. In particular, if you have enough gold for your other projects, I recommend shutting down your bakery and your tool shop, as they have very low experience point production.