Sorry - just to add - i am sure you may not need one if you are going out to hour build?
I think the M1 requirement is for training purposes the last time i checked.
I would go out under the visa waiver programme - just say holiday as your reasons for entry and do your hour building on the sly.
If you tell them hour building expect to be hauled aside for 4 hours and treated like osama's 2nd cousin.
Just a thought - save a lot of hassle - although things might have changed since.