The promise: your city keeps working while you do not
BuildCity is designed around short daily sessions. The pitch on the home page is literal: the city keeps growing while you are away. Taxes accumulate, construction timers run down, and when you come back the game shows you a report of everything that happened. The design question we had to answer early was how long "while you are away" should be allowed to pay out, because an idle game with no cap simply rewards whoever started earliest and never logs in.
This article explains the rules as they stand today: how idle income is calculated, where the 24-hour cap comes from, why the second day is worth half, and how the October 2026 settlement rework changed what the server does when you return.
How idle income is calculated
Every city has an hourly tax rate derived from its buildings: residential tax scaled by residents, happiness, and the density curve; commercial and industrial tax from building counts and their density multiplier. When you leave, the server records the time. When you return, it measures the elapsed time, multiplies by the hourly rate, applies the time weighting described below, and credits the result.
The first 24 hours away are paid at full rate. Hours 24 through 48 are paid at half rate. Anything past 48 hours earns nothing extra. A 36-hour absence therefore pays 24 + 6 = 30 hours of income, and a week away pays the same 36 hours as a two-day absence.
Not every building contributes evenly across the clock. Commercial buildings have operating hours, and the hours when shops are closed integrate to zero rather than being backfilled from an average. If your economy leans heavily on commercial tax, your overnight idle income is noticeably lower than your daytime rate, and that is by design rather than a bug.
Why 24 hours, and why the second day is half
The cap exists to make returning worth something. If a city paid the same per hour forever, there would be no reason to open the game more than once a week, and the social parts of BuildCity, from rankings to sister cities to visiting, depend on players actually being around. A hard cliff at 24 hours felt punishing in testing: miss a day by an hour and lose a whole cycle. The half-rate second day is the compromise. It still rewards a daily rhythm, but a late return is a smaller loss rather than a zero.
The number itself is a pacing decision. A full day of income is roughly the amount a mid-sized city needs to fund its next meaningful upgrade: a large service building, a utility expansion, or a block of new zoning. Capping at one day keeps each session about one decision, which is the loop we want: log in, collect, fix the current bottleneck, leave.
What the return report shows
When you load a city after being away, the report modal summarizes the period. It lists income earned, upkeep and other expenses paid, buildings that finished construction, buildings that were abandoned because their happiness stayed at zero, and any disaster damage that occurred. The expenses line matters more than people expect: service upkeep, add-on costs such as school classrooms and bus depots, and warehouse truck costs are all charged for the same weighted hours as the income, so a city that is barely profitable online can come back from a day away with less money than it left with.
Disasters are the other surprise. Fire trucks and police do not respond while the city is unloaded, so a fire that starts during an absence runs to completion and its damage is applied in full. Coverage still matters, because a cell inside a fire station's radius takes reduced damage even without an active response, which is one reason the coverage article argues for radius over reaction time.
The October 2026 settlement rework
Until this autumn, the client and the server each computed parts of the offline period independently, and they did not always agree. The server credited the pre-tax-rate base amount and did not subtract upkeep. The client recalculated taxes on load and added them to the displayed balance, which produced moments where a returning player saw more money than the server would ever persist. We documented the mismatch in detail while designing the late-game economy and then fixed the accounting before touching any balance numbers.
The server now keeps a protected economic ledger inside the saved city. Every action that changes cash, from loading the city to claiming a daily reward to receiving a sister-city gift, first settles the elapsed period: taxes at your chosen rate, facility upkeep, add-on and truck costs, and the new city operating cost are applied once, in order, with fractional remainders carried forward. The browser uses the same period boundaries and the same remainders for its prediction, so the number you see on return is the number that is saved. A period that was settled by the server is never paid again by the client.
Two deliberate limits were kept. The server still caps accumulated income at 24 weighted hours, and no past period that was already settled under the old rules is re-billed under the new ones. The city operating cost introduced alongside this change phases in over your first 72 effective hours of play after the update, at 25, 50, 75, and then 100 percent, so an established city does not wake up to a sudden bill.
The rest of the daily loop
Idle income is the backbone, but the daily session has three other sources of progress that reward showing up rather than waiting.
- Daily rewards run on a seven-day cycle that loops back to day one after day seven. Missing two days in a row resets the cycle. Money rewards go to the city you last played; gem rewards go to your account.
- Daily quests are three fixed goals per day: grow population by 100, add five buildings, and reach 80 percent happiness. They measure progress from a snapshot taken when the quest was created, so they are about today's play, not your lifetime totals.
- Construction timers keep running while you are away, so queuing a long build before you leave turns idle time into finished infrastructure.
Achievements sit on top of all of this and are evaluated against the best value across all of your cities, which means a second city never costs you progress on the first.
Planning your check-ins around the cap
- Spend before you leave. Cash sitting in the treasury earns nothing; a queued school or utility upgrade does.
- Fix utility shortages first. A power or water deficit applies its happiness penalty for the entire absence and drags down the tax rate for every hour of it.
- Leave fire and police coverage in place over your densest blocks. Offline disasters are not fought, only mitigated by radius.
- Return within 24 hours when you can, and within 48 when you cannot. Beyond that, the city simply waits.
- Read the expenses line in the return report. If it is close to the income line, your next investment should raise land value, not add more upkeep.
The Taxes and Budget guide covers the online side of the same economy, and the tax curve article explains how the hourly rate itself is computed for dense cities.