House training
Why did my dog start marking indoors after I got a second pet?
Last updated 23 September 2026 · How we write and check this
Short answer
A new animal in the home introduces unfamiliar scents throughout the house, and marking is a common way an established dog responds to that change, asserting their presence in spaces that suddenly smell different. This usually settles within 3 to 6 weeks as both animals adjust, especially with supervised, gradual introductions rather than sudden full access.
Why a new pet triggers this specifically
The resident dog's sense of the home as entirely their own territory shifts the moment a new animal's scent appears throughout shared spaces, and marking is one of the more common ways dogs respond to that disruption, even dogs who have never marked indoors before.
Easing the transition for both animals
Introduce the new pet's scent and presence gradually rather than giving them full run of the house immediately, and supervise the resident dog closely during the first few weeks, redirecting any pre marking posture outside promptly and cleaning any marked spots thoroughly.
A practical first month plan
In week one, keep the new pet's bedding, toys and food area in a separate room so the resident dog can investigate the scent without direct access to the objects themselves, which lowers the intensity of the reaction compared with everything appearing in shared space at once. Swap bedding between the two animals for a few nights around this stage so each gets used to the other's scent gradually.
From week two, allow short, supervised time together in a neutral space, ten to fifteen minutes at a time, building up gradually rather than granting full free roam access on day one. Watch the resident dog specifically for the stiff legged pause and slight lean that usually precedes marking, and interrupt with a calm redirect outside before it happens rather than after.
Most households see marking drop off noticeably by the three week mark once the new pet's scent has become familiar rather than novel throughout the home, though a dog with a strong prior marking habit may take closer to six weeks. If it has not eased at all by then, treat it as a separate marking habit needing its own training plan rather than assuming time alone will finish the job.
Key takeaways
- Marking after a new pet arrives reflects a response to unfamiliar scent throughout shared spaces.
- It usually settles within 3 to 6 weeks as both animals adjust to each other.
- Gradual introductions and close supervision during the first weeks ease the transition faster.