Completing community buildings increases your town's population cap and allows you to build more houses.
Once the construction time of a building has elapsed, the building will be wrapped up with balloons if you have the required materials in your barn to finish construction, at which point you may choose spend those materials to complete it. If you are missing materials, the building will be surrounded with wood scaffolding with a yellow construction helmet icon floating above. To see what materials you'll need to finish each building, tap on the construction helmet.
Some buildings have animated features (just like factories when they are working). They can be simple like the Post Office man checking his watch, while others are more complex like the Roller Coaster or Fashion House. Visit Ernie or higher level friends to see which buildings will animate in your town, or refer to the table below.
NOTE: If you build more than 1 community building at a time it may throw off the train algorithm and bring you extra unwanted materials. You can read more about correcting this here.
Complete the community building with the lowest amount of materials required first, then continue in that order until you are back on track.
If you build more than 1 community building at a time it may throw off the train algorithm and bring you extra unwanted materials from Trains.
Avoid using the Tool Exchange more often.
Having too much of 1 type of construction material from trains can result in getting too many of the same material. It's best to sell materials if you have too many of one type. Find the lowest amount needed to finish a community building or barn upgrade and sell the materials/tools down to 1 below what's required to complete the building or upgrade.
Never exchange materials that you require to build community buildings with tools, in the tool exchange
Using T-cash to buy materials is quite expensive. It's better to use trains instead
From level 60, new Community Buildings require special tools that cannot be obtained from trains. These tools can only be produced by a special building, called Tool Exchange. The Tool Exchange produces jackhammers, drills and electric saws.