Your dog is on the sofa. You go to sit down, or move them, or reach past them, and there is a stiffening, a hard stare, a low growl. Sometimes it happens when they are lying against one particular person.
Read this part first
A growl is information, delivered early and at no cost to you. It is your dog saying they need the situation to change before it goes further.
This is the one adolescent problem we do not hand you a training plan for. Guarding a place or a person carries a risk of a bite to someone in your household, and it needs eyes on your actual dog.
Safety note
Get help now. A dog who has growled at, snapped at or bitten a person needs a veterinary examination and a qualified behaviour professional. An article is not enough, and neither is a plan bought online. Book the veterinary appointment first, because pain frequently presents as a change in behaviour rather than lameness.
What to do between now and that appointment
Stop moving your dog off furniture by hand, and stop reaching over them. Every one of those is a rehearsal you do not want.
Call your dog off the sofa instead, from 4 feet away, and pay them with food on the floor when they come. Getting off should be their idea and it should pay.
Keep children away from your dog while your dog is resting, resting on a person, or on furniture. No exceptions, and no supervised exceptions.
Write down each incident: the date, who was there, what happened immediately before. The behaviour professional will ask, and your memory will not be reliable.
If it is one piece of furniture, make it unavailable for now with a laundry basket or a closed door. Removing the contested thing removes the contest.
What not to do
You will read that you should assert yourself, or that your dog is trying to outrank you. Ignore all of it. A review of aversive training methods found them associated with worse welfare outcomes and no training advantage.
Punishing a growl is the specific mistake with the worst outcome. Punishment-based approaches are associated with fear, avoidance and defensive aggression, and a dog who learns growling is unsafe may go straight to the next thing.
Why adolescence is when this shows up
Dogs respond less to the cues of the person who raised them around the onset of adolescence, and responsiveness returns later in the stage. Behaviour that was easy to interrupt at 5 months old is harder to interrupt at 11 months old.
That does not make guarding a phase to wait out. It makes now the right time to have somebody qualified look at it.
Today's version, in one line: nobody reaches over your dog, and the veterinary appointment is booked.