Is there anywhere else you can store things for a bit? Maybe take a load of bags of clothes to a friends or family members house and ask if you can store it there for a week or so whilst you sort your house out?
It sounds like everything stored in the attic is being pushed into an already full house to make way for the office? If this is true, then you may want to consider how much stuff you have and get rid of everything you don't completely love or isn't useful.
I have recently done a massive de-clutter of our house (it was so bad) and the men at our local rubbish dump got to know me very well as I was there every other day with a car full of black bin bags to throw out.
I think the best thing I did was unplug the TV and stack bags of stuff on top of the sofas so that we couldn't just sit down and veg in front of the TV. It forced us to get on with the job each evening rather than ignoring it all and watching TV.
My method of de-cluttering (which may work for you as we also had very limited space) was this:
Firstly, I packed a bag so that I had everything I needed for a week, and did the same with everyone who lives at our house. I put these bags somewhere easily accessible.
Then I invested in lots of those big see-through 65 litre SAMLA boxes with lids from IKEA, and sorted a room at a time. link
Everything I wanted to keep went into those boxes. A box for clothes, a box for stuff I want to keep, a box for stuff that someone else has to decide on, a box for stuff that lives in a different room of the house, or has no home yet. Then I stacked up those boxes against the wall and moved onto the next room.
Every day the rubbish went into the car and when it was full I took it all to my local tip. Very satisfying!
I also identified larger items that I wanted to get rid of that could be donated, and put it all in the car at the beginning of my decluttering to make space quickly. I did one big run of donations to the various places that would take them. Also, don't feel bad if you don't have time to donate things, just throw them instead. Our local tip has a re-use shop that takes donated items, so maybe yours does too?
And lastly, I decided where everything was going, made sure I had homes for all the things I had decided to keep (I stuck post it notes on every drawer, box and cupboard listing what items were 'allowed' in there), and then took everything out of the SAMLA boxes and put it away. If I hadn't given something a home, then it went back into the box for later. At the end I looked at all the stuff I hadn't given a home to, decided to throw some of it out and made homes for the other stuff. Nothing got out of those boxes without a home to go to! I kept the post-it notes on the storage areas for about a week to help remind me where each item went.
Then I made sure that everyone understood the rule 'you take it out you put it away', and now I always make sure clutter is put away before bed and all the kitchen items go into the dishwasher/get washed up before bed.
If you find you don;t have enough homes for items, then I can recommend IKEA storage systems (such as the KALLAX range which will store loads of stuff in a very efficient way, for a very reasonable price.
I hope you make some more progress and keep throwing stuff out, you will make progress as long as you keep at it! 