Cornwall is unusually good for dogs in some ways and unusually difficult in others. The coast path is open to dogs the entire length of the county, the off-season weather is mild enough for year-round walking, and most cottages along the coast now welcome them properly. The complication is the summer beach bans — between Easter Sunday and 1 October, many of the headline beaches (Porthminster, Porthmeor, Towan, Summerleaze, Padstow town beach, Polzeath in part) operate dog restrictions during the day. If your dog is the reason you're coming, time the trip for October to April, or pick a base with at least one year-round beach within walking distance.
The cottage criteria that actually matter, in order of impact: an enclosed garden you can let the dog out into without putting the lead on; hard floors that survive sandy paws; a downstairs cloakroom or boot room to manage the wet-dog routine; a kitchen door to the garden rather than through the lounge; and — easily forgotten — a sensible distance from the road for early-morning loo trips. Premium dog-specific cottages add fenced paddocks, dog-washing showers, branded towels, and even a freezer compartment for raw food. The price gap from "accepts dogs" to "designed around dogs" is usually £200-400 a week — and worth it for longer stays.