SN, yes we WILL be getting the builders in to do the actual work, but DH wants to have his arm back out of plaster before they start. Which I can understand as we have lived around builders before and it IS doable, but we need to be in the whole of our health to do it.
But we decided to get the unit now anyway, and get started on the process as we have SOOO much clutter to get down and sort through in the first place. I am going to get rid of a lot of out-of-season clothes too, and the clothes that no longer fit anyone (mostly DD's but there are a few of ours too).
Our plan is that, while some stuff will have to come back into the house after the work, a lot of it shouldn't. So we will sell, donate, give to family - whatever we can - a lot of the boxes of things that we don't and won't use. Which is a lot easier to do when we are not falling over everything.
And yes, the attic will become an extra room - we are still debating whether it will be a study/office/craft room or an extra bedroom (and have the study etc in the current box-room).