Universal Credit switches from a single claim to a household assessment
If a partner moves in, Universal Credit is usually no longer assessed just on you. The system starts looking at the household as a couple instead. That changes the standard allowance, but it also brings the other person's earnings and savings into the same calculation.
The couple allowance is higher than the single allowance, but that does not guarantee a higher final award. In practice, the new partner's income is often the bigger driver.
Savings and earnings are combined
Couples are assessed on joint capital. A partner's savings can therefore move the household into the tapered range or above the usual capital limit even if you had little saved yourself.
The same is true for wages. A partner moving in can change the whole result because their net earnings become part of the same assessment.
Treat this as both a reporting issue and a planning issue
A partner moving in is one of the clearest examples of why this site is built around connected pages rather than a single headline estimate. Child Benefit, HICBC, council tax help and childcare support can all change when the household changes.
Use the calculator to understand the direction of travel, but report the change promptly to the official service once it becomes real.