An irrigation lateral underground

The most common finding on this coast, and the hardest to notice, because the system still appears to work. Look for a patch of landscaping that is greener than the rest.

A toilet running intermittently

A flapper that seals imperfectly cycles quietly and can account for a surprising share of a step. Worth eliminating first because it costs nothing to check.

A weeping fitting inside a wall

Small, continuous, and the one that eventually becomes a restoration job. This is the case where the bill has been telling you for months.

A service line between meter and house

Confirmed by the shut-off test above. The repair is a different trade and often a different conversation with an insurer.

Reading a water bill for a leak you cannot see

In a place where it does not rain for most of the year, the water bill is the closest thing a house has to a continuous witness. A leak on the supply side of the meter is water the utility delivered and measured, whether it ended up in a sink or in the ground under a foundation. That makes the billing history a record of something nobody watched happen.

Read it as a series, not as a number

A single month tells you almost nothing, because usage moves with the season, with guests, with a new garden. What is diagnostic is the shape of the line over a year or two. Normal variation goes up and comes back down. A leak does something different: consumption steps up to a new level and stays there, month after month, including the months when nothing about the household changed.

The step is often modest. A weeping fitting behind a wall does not double a bill; it raises the floor of it. That is why the pattern matters more than the size — a small permanent increase is more suspicious than a large temporary one.

Confirming it at the meter

The meter itself settles the question in about twenty minutes. Turn off every fixture and appliance that uses water, including any irrigation controller and any ice maker. Find the meter — usually in a covered box near the street — and look at the small dial or triangle that registers low flow. If it is turning with everything off, water is moving somewhere.

To find out whether it is inside or outside the house, close the main shut-off at the building and watch the meter again. Still turning means the leak is between the meter and the house — a service line or, in this region very commonly, an irrigation lateral. Stopped means it is inside.

That single test separates two completely different repairs, and it can be done before anyone is called.

What it does not catch

Anything on the drain side. A failing shower pan, a cracked drain joint, a toilet flange that has been weeping into a subfloor for a year — none of these consume extra water, because the water was going to be used anyway. They are among the most common slow leaks in older houses, and the bill is silent about all of them.

It is also silent about water arriving from outside the building envelope, and about condensation, which never passed through a meter at all.

What a step in the line usually turns out to be

An irrigation lateral underground: The most common finding on this coast, and the hardest to notice, because the system still appears to work. Look for a patch of landscaping that is greener than the rest.
A toilet running intermittently: A flapper that seals imperfectly cycles quietly and can account for a surprising share of a step. Worth eliminating first because it costs nothing to check.
A weeping fitting inside a wall: Small, continuous, and the one that eventually becomes a restoration job. This is the case where the bill has been telling you for months.
A service line between meter and house: Confirmed by the shut-off test above. The repair is a different trade and often a different conversation with an insurer.