Q:

how should i approach my boss with my resignation letter? 

So the time has come for me to hand my notice in, as I'm going back to the company I left 6 months ago returning to a higher position making more money and with a good training program set out for me. 
This same week the new building has been finished, and I am being told about my office moving into the new building.  They want to start making plans to get me moved by the end of the month
I will be handing my notice in on Friday and I'm not sure how to deal with the situation as I can see this becoming an issue with everything they are starting to put in place. 
I want to give an extra week of notice (2 weeks instead of 1 as I'm in the probationary period) because I don't want to just drop it on them so to speak
However, I'm not sure on how to to tell my boss as I feel guilty knowing I am leaving as they are making plans for me 

A:

Approach it like any other resignation
There, that's the simple answer.  I'll go into detail below. 
I'm assuming you are some sort of team leader/manager from your comments. 

although this same week the new building has been finished and I am being told about my office moving into the new building and they want to start making plans to get me moved by the end of the month
I will be handing my notice in on Friday and im not sure how to deal with the situation as I can see this becoming an issue with everything they are starting to put in place. 

Moving offices is purely a logistical exercise.  It doesn't seem to me, from what little we have been told, that you leaving will have a major impact on moving people to different desks in a different building.  The company will have plans in place and probably know where everyone is going to sit anyway.  Or if this up to you, you will have done this already. 

I am giving an extra week of notice (2 weeks instead of 1 as im in
probation) as I don't want to just drop them in it so to speak
but im not sure on how to to tell my boss as I feel guilty knowing im
leaving as they are making plans for me

I don't want to sound harsh but you clearly haven't been there long enough to truly drop them in it.  Otherwise they would have got you off probation and tied down to a longer notice period.  I'm guessing you aren't director level as you are moving for a promotion, so I am guessing while your departure may set them back, it will probably be able to
Don't give the extra notice.  What more could you achieve in 1 week?  Just make sure any loose ends are tied up and anything you can't do in that time are documented clearly so whoever takes your role on can do what they need to do.