Short answer
Set Plumber as your primary category unless the majority of your revenue is drainage or gasfitting. Then add secondary categories that match work you genuinely do, such as drainage service, gasfitter or hot water system supplier. Each one opens searches you are currently invisible for.
Categories are the single most under-considered setting on a plumber’s Google Business Profile, and they do more work than almost anything else on it.
What does the primary category actually do?
It tells Google what kind of business you are, and it carries far more weight than any of the secondary ones. It is the main thing deciding which searches you are even eligible to appear for in the map.
For most plumbing businesses the answer is simply Plumber. Do not overthink it.
The exception is when the bulk of your revenue comes from something more specific. If four fifths of your work is drainage, Drainage Service as primary with Plumber secondary may serve you better. The test is what you actually want more of, not what your business name says.
Which secondary categories are worth adding?
Add the ones that describe work you would genuinely take tomorrow. Commonly useful for plumbers:
- Drainage service
- Gasfitter
- Hot water system supplier
- Bathroom remodeller
- Water heater repair service
- Septic system service, if you do rural work
Each one opens you to searches you are currently invisible for, at no cost. That is the upside, and it is real.
The mistake worth avoiding
Filling every available slot.
A category you do not really service produces calls for work you turn down. Turned down calls waste your time, and people who feel messed about leave reviews saying so. You end up paying for the wrong visibility twice over.
Pick honestly. Three accurate secondary categories beat eight aspirational ones.
How do I know what my competitors use?
You can see the primary category on any public profile, and it is worth spending twenty minutes looking at the three plumbers currently sitting in your map pack. Not to copy them, but because it tells you what Google currently thinks a plumber in your town looks like.
If all three use Plumber and you are using something niche, that is worth knowing.
What to do after changing them
Leave it alone for a few weeks.
The primary category is a heavy ranking factor, so changing it moves things. If you change categories, service areas, hours and your description all in one afternoon, you will have no idea which change did what. Make one meaningful change, wait, then look.
This is the same discipline as everything else on the profile: one change at a time, and something measuring the result. If nothing is tracking your calls, you are guessing either way.