From the outside , I'm always slightly surprised how Sheffield United supporters seem to sell themselves short in terms of the size and potential of their club ..a stark contrast to your neighbours perhaps ?
Sheffield United attendances have been extremely healthy for many seasons now , and Bramall Lane is always close to capacity on match day ..this season 28,746
Compare that to other clubs with 30,000+ capacity similarly sized stadiums ..Blackburn , Birmingham , Cardiff , Middlesborough , Coventry , Stoke etc ..none of these clubs come close to matching Sheffield United's turnout .not just in a good year , but any year
If you come close to averaging 30,000 in the Championship ..and that's with away support from the likes of Reading , Wigan , Luton and QPR , then quite clearly you could do an extension on 32,000+ when in the Premier League
I don't know how you do it , but it's obvious , if the capacity was 35,000 for the Premier League .it would not be a problem to fill it