We had a quote last week for ours (in Brixton not East Dulwich) and it was about 45k for the basics by Buildteam - who seemed super professional - but we are worried about the reviews they have had on mumsnet etc. However I've spoken to lots of our neighbours and 40-60k seems about average. I think side returns seem quite overwhelmingly complicated but when we realised lots of our neighbours had had them done we thought it can't be that bad! We heard on here that Prestige builders were really good but they wouldn't come and look at the job and were pretty uninterested on the phone. So I'm still looking for some other quotes...