Range from the cheapest Side return extension £32,500 (Ghani Constructions based in IG1) to the dearest Side return extension £40,870 (Maconti Design 3000 based in E12).
Comparing extension builders estimates for Side return extension gives you an idea of who will be better value but the best way to compare costs is to get actual quotes.
What to do - Get rates and quotes for a side return extention by posting your job to extension builders in Redbridge.

"Side return extension"
REDBRIDGE