Hi Lord, I've posted following link somewhere in here before -
it might be worth to try to check following pages, it is UKBA Entry Clearance Guidance issued to their officers to help them to assess the applications. One of the parts contains guidance how to assess the accommodation issues.
Please see this:
UKBA Entry CLearance Guidance - Accommodation
Especially pay attention to the paragraphs MAA11 to MAA14 - according to this guidance you should be OK as your house is apparently 5 bedroom house so it is allowed to house 10 people (that one of your bedrooms is basically your office is none of their concern) and as I've counted there will be 7 people living in there including your wife and kid (and your kid counts only as a HALF person!)
Good luck
