A lot of people ask whether they should overseed their existing lawn or tear it up and start from scratch, seeding a new lawn. A basic rule of thumb is take a good honest look at your lawn. Is it more than 50 percent bare spots and weeds? if it is, you're probably going to need to start from scratch and create a new lawn. That's a lot of work though and sometimes may be best left to the professionals.
It's always best to overseed your lawn if you can, it's less work. Because if you're going to start with a fresh lawn and a whole new grass system, you're going to have to find out something to do with the sod because that's going to have to be taken away.
And then lastly the thing to remember is that you're not going to get rid of the weed seed bank by putting a new lawn. There's still weed seeds in the soil and they're going to sprout with the new grass seed so if you thing you're getting rid of weed, you're probably not going to. You're still going to have to deal with those anyway.
So if you can, try to renovate your existing lawn. But if you have to, if it's more than 50 percent weeds and bare spots, think about starting with a new lawn.