EXERCISE
Just because you live in a flat without a garden does not mean you cannot own a dog. This will just mean that you have to be even more dedicated to walk your dog as you do not have a garden to let him out in.
Owning a dog means you cannot be a 'fair weather' walker. Dogs like routine and soon tell you when it is time for their walk, whatever the weather conditions.
How much exercise to give your dog depends on the breed, age and your own personal circumstances. It is also a fallacy that because it is a small breed it will not need a lot of exercise. Many of the small terrier breeds were bred for hunting and even a whole day out will not tire them out. Care should be taken though not to over exercise a young dog. While a dog is maturing its bones are still developing and serious damage can be done to joints and bones if over exercised. A puppy should not be allowed to jump on and off chairs or be allow upstairs for the same reasons.
You may be lucky to own acres of land for your dog to roam freely, but is this enough? A dog is a sociable animal and loves meeting and playing with other canines.
If you have the space, but not a lot of time to exercise, then the Greyhound may be the ideal dog for you. There are many ex-racing Greyhounds that need rehoming and these dogs generally settle happily into a home environment and can become real 'couch potatoes'.
On the other hand, should you have lots of free time to walk a dog, then the Border Collie would be an ideal companion as this working breed needs plenty of exercise and also makes an ideal agility dog.