The consequence for dogs of showing aggression towards people is often euthanasia or relinquishment. Aggression is also a sign of compromised welfare in dogs, and a public health issue for people. The aims of this study were to estimate the numbers of dogs showing aggression to people in three contexts (unfamiliar people on entering, or outside the house, and family members); identify whether these co-occur, and investigate risk factors for aggression in each context using multivariable analyses. In this cross-sectional convenience sample of UK dog owners, aggression (defined as barking, lunging, growling or biting) towards unfamiliar people was more common than towards family members. Most dogs did not show aggression in multiple contexts, suggesting that this behaviour may be a learnt response to situations rather than a general characteristic of individuals. Older owners were less likely to report family directed aggression or aggression to unfamiliar people entering the house than younger ones. Female owners were also less likely to report aggression to visitors. Increasing dog age was associated with increased risk of aggression to unfamiliar people both entering and outside the house. Female neutered dogs had a reduced risk of aggression in all three contexts. Dogs in the Utility and Hounds groups as defined by the UK Kennel Club had an increased risk of aggression to family members compared to cross-breeds, although post hoc analyses identified no specific increased individual breed risks. Gundogs has a reduced risk of aggression to unfamiliar people both entering and outside of the house. Where owners acquired their dog was a risk factor for aggression to household members. Attendance at puppy classes reduced risk of aggression to unfamiliar people both in and out of the house; attending ring-craft classes were associated with reduced risk when outside the house. The use of positive punishment or negative reinforcement based training methods was associated with increased chance of aggression to family and unfamiliar people outside the house. Importantly, for all types of aggression, the variables measured explained a relatively small amount of the variance (<10%) between aggressive and non-aggressive animals, suggesting a much greater importance of factors specific to the experience of individual dogs in the development of aggression. These data suggest that although general characteristics of dogs and owners may be a factor at population level, it would be inappropriate to make assumptions about an individual animal's risk of aggression to people based on characteristics such as breed.