If you have any mods enabled, it will disable your ability to get achievements. If you have a city that you started using mods on, then that city is "dead". Your city must never have the taste of mods if you want achievements. I suggest having a separate city for achievements and naming it something so that you know to disable mods when playing on it.
This achievement is pretty easy. But, can be hard to handle if your city is kind of small.
1) Set everything other than residential to 11% tax rates. (Low Density Commercial, High Density Commercial, Industrial, and Office)
2) Set all residential taxes to 6%. (Low Density Residential, High Density Residential)
3) Wait 1 full year.
That's it!
Some things to keep in mind:
- You can change the taxes by clicking on the stack of money on the bar at the bottom and then clicking on the first tab in the new window.
- Depending on what your taxes were at before, you could see quite the hit on your weekly income.
- Feel free to adjust those values as you see fit. But, make sure that none of them are less than 5% from each other at any point during the year period.
- The bar in the bottom left that keeps filling and depleting is your progress bar. Each time it resets, you've gone one full day.
- Take a note of what day-ish you started on. Then check Steam (or here) when it has passed to make sure you can change your taxes back.
Happy hunting!