Cross-subsidies for improved sanitation in low income settlements: Assessing the willingness to pay of water utility customers in Kenyan cities

•The Kenyan water sector regulator is exploring sanitation surcharges to benefit the poor.•We measured water utility customers' willingness-to-pay (WTP) for these surcharges.•We found a mean WTP of USD 2.90 per month and a median WTP of USD 1.00 per month.•WTP was higher among customers that reported trusting their water utility.•Sanitation surcharges have the potential to generate revenue to improve sanitation.