If you have a dog, it'll be peeing on your grass, a classic cause of dead brown patches in turf. You can try buying something called dog rocks, available from vets, to put in the drinking water, which will help to neutralise the urine. Otherwise, you can try flushing the area as soon as the dog has used it with the hosepipe to reduce the effects. This might not be possible if you live in the south or east after 5th April though because of the hosepipe ban.
If you can solve the problem of the dog urine, then its worth reseeding - if you can't, waste of time.

the thing is it wont just be the peeing it will be wear the dog runs about on the grass etc . the best realistic cure is to have a dog run wear they can go to the loo realy .

Reseed with a rhizomatous grass, such as bluegrass or creeping fescue. If you only have a clumping grass, such as perennial ryegrass, tall fescue, or Chewings fescue, those brown spots are forever. If you reseed to repair the damage, the next time the pooch traipses over the spot, he will just drag the seed, cover, and mud into the house when he comes back in. Rhizomatous grassses repair themselves from underground, so just keeping them well fed and mowed frequently will allow the spots to gradually disappear. Wear trails are another matter, and sometimes the only thing to do is adopt them as your paths, and lay flags! Keeping the grass happy and healthy with good culture will make all kinds of damage less....In fact, it might be a good idea to double check your cultural practices, just in case it isn't all the dog's fault.