Can Ants Nest Inside Walls and Under Floors?

The Normal Case: Outside

Black garden ants, which are the overwhelming majority of our domestic calls, nest in soil — under paving, at the base of a wall, under a step, in a border or a lawn. They come inside to forage and go home again.

So a trail across the kitchen is not evidence of a nest in the kitchen. It is evidence of a road, and the road has two ends.

When They Do Nest Inside

Warm dry voids with a way in and a food supply within reach: a cavity wall, particularly against a south-facing elevation; under a suspended floor; behind boxing near a boiler or a hot pipe; in wall insulation; and occasionally in an airing cupboard.

A nest under a conservatory or a solid floor is common and sits in a grey area — technically outside the building, in the sand or hardcore beneath the slab, but functionally indoors.

How to Tell the Difference

A trail from an outside nest goes somewhere. Follow it in the warm part of the day and it leads to a door threshold, a window, a pipe entry or a crack in the wall, and continues outside.

An indoor nest does not. The trail emerges from one point and disperses, activity continues in cold or wet weather when outdoor ants would be quiet, and you may find winged ants indoors well outside the usual July flying-ant window. Fine debris or soil appearing from a crack indoors is the strongest sign of all.

The Species That Genuinely Live Indoors

Pharaoh ants are a different proposition entirely. They are very small, yellow-brown, tropical, and they nest in warm building voids all year round — hospitals, blocks of flats, anywhere heated continuously. They do not need to go outside at all.

They also matter because spraying them causes budding: the colony splits and scatters into new nests through the building, turning one problem into several. If a small yellowish ant is active indoors in midwinter, it should be identified before anything is applied to it.

What Changes About the Treatment

The principle is the same — slow-acting gel bait carried back by the foragers, never a spray — but the placement changes, and an indoor nest means treating the void rather than the route.

Which is why the survey comes first. Ant treatment is £190 for a flat and £220 for a house, and part of what that buys is somebody establishing which of these you actually have before putting anything down.