I've never marketed for a plumber but the basic fundamentald for any type for service based business should be met before any kind of one is spent on ppc.
First make sure the business is listed on all the local search directories. These include google, yahoo, bing, yelp and yellowpages to name a few. Make sure the phone number, link back to the business website, hours of operation service areas and address for the business are correct. While you're at it.. gather a list of previous clients and ask them a series of survey questions, short and sweet, that outline what they enjoyed most about the services, how we could improve, would you recommend? If so, here is a link to yellowpages reviews and facebook shares. Any opportunity for free good word of mouth from a happy client should be used.
Once this has been taken care of, you will already have a whole bunch of backlinks to the site.That should generate a bit more traffic. But more than anything else, these are all customers visiting yellowpages for a plumber already searching for these services. They will see happy good reviews, correct information, clean backlinks and feel a sense of organization before they even contact you. Potential customers may not even need to directly visit the site to hire them. Maybe write a few keyword optimized articles about a few different subject that are somewhat searched for.
Next you'll want to really focus on some keyword optimization on each page if the site. The hard part is picking the right keywords. Some ideas for picking keywords, imo, would be maybe; the city the services are being offered and some particular services that this plummer does that's not too often thought of unless they need the service.
"leaky faucet repair in burbank"
Or
"Copper repipe services Los angeles"
Or
"Water heater replacement Torrence"
come to mind for some reason.
While these might not have the insane numbers of people searching for them, these are pretty specific searches and most likely, If someone is looking for these terms they are either looking to do it on their own or looking to hire someone to do that particular job.
Lastly, take advantage of some social media, plumbing is not the most elegant market to be posting insagram pics, but people on Facebook could share their brand new shower or tub they just put in.
I hope this helps! Remember take initiative! Don't sit around reading articles till times passed you by!