Can my neighbour build a shed against my fence?

Permitted development doesn't require a gap between an outbuilding and the boundary. The trade-off is height: within 2 metres of the boundary, the whole building is capped at 2.5 metres. A shed hard against the fence line at that height is, in most gardens, entirely lawful.

The boundary itself is where their rights stop. If the fence is yours, they can't fix a shed to it, lean structures on it, or use it as one wall of the building. Guttering, roof overhangs or felt lapping over the line onto your side is a trespass, however small.

If the shed breaches the outbuilding rules — taller than 2.5 metres near the boundary, covering too much of their plot, or in front of the house — it needs planning permission, and you can report a suspected breach to the council's planning enforcement team, who will investigate.

Note what isn't a planning matter: losing the light to a flower bed, a shed being ugly, or the fence dispute you already had. Enforcement acts on rule breaches, not aesthetics or neighbour history.

If you're the one planning the shed, the same rules apply in reverse — and a friendly conversation before the slabs go down prevents most of the disputes that end up in the council's inbox.