Some tips for the Construct-O-Coins mission
To start the CC mission, you need the latest sims update.
The best way to get CCs is to get your sims to do woodworking as a hobby. To do this, you need the community centre (top corner, the logo is a green circle with orange centre, with three people on it).
Every woodworking task the sims complete, they will get some number of CCs for the finished product. The sims can build all products unlocked by the level of the most skilled sim. So the best strategy is to focus on building up at least one sim’s skill level to 6/6. Then, get three sims to do a “quick practice” every 4 mins for lots of CCs!
Items your sims produce are worth between 1 and 10 CCs (for the level 6 items). You also get bonus CCs for completing sets of items: this goes up to 250CCs if you complete all of the sets.
Using your CCs to add floors: it costs 220CCs to add a 2nd floor, and 300 for a 3rd, regardless of house size. It essentially builds an exterior wall that matches the ground floor house plan (so your house is twice as high), inside of which you can add rooms.