We have an undermount sink in wood worktop and i can't find anything that says the overhang should be more than 10mm but builders merchant who supplied it has advised our builders to allow 25mm - i think this is too much but as i'm just the householder it's a difficult thing to query without looking like i'm questioning their wisdom (?) if ths was the first issue like this to come up then i wouldn't worry about asking but there have been numerous quality issues with the builders over the course of a 6 month build and i'm getting tired of checking stuff - but it's still keeping me up at night though 
