Search for the street in the 1911 census, if you cannot find it then you are post 1911. http://search.findmypast.co.uk/streets/street-results-for-1911-census-for-england-and-wales?addressstreet=southwell%20road
Follow that with some style features from the valuation office and the sash windows probably put you into the early 1920s.
http://manuals.voa.gov.uk/corporate/publications/propertydetailsguide/ageCodes.html
I'd probably put 1924 down or ask an owner occupier neighbour if they know.
Wait what, that is clearly a Victorian house, it's obviously going to be there pre-1911 or did I miss something?
My reasoning isn't as advanced as BQ's but I looked at it and would have guessed 1850-70.
@Ramsaye started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Search for the street in the 1911 census, if you cannot find it then you are post 1911.
http://search.findmypast.co.uk/streets/street-results-for-1911-census-for-england-and-wales?addressstreet=southwell%20road
Follow that with some style features from the valuation office and the sash windows probably put you into the early 1920s.
http://manuals.voa.gov.uk/corporate/publications/propertydetailsguide/ageCodes.html
I'd probably put 1924 down or ask an owner occupier neighbour if they know.