u587162, Hi
You are entering a mirky area which is full of huge financial pitfalls.
If you are Un-Lucky to have seen some of the TV programmes about large value and very fancy new builds and re-furbs with all of the pitfalls of the [if you excuse the expression] well meaning home owner taking on the roll of a Project manager without any previous experience is a really, really daunting task.
You will need, a set of highly detailed plans from your Architect right down to the ironmongery, type of windows, door types, grade and exact type of flooring, Carpets. wall types, finishes, papered or painted, types of Skirting. the kitchen, and the bathroom, just do not go there the options are totally mind blowing, as you possibly know already.
Now imaging part way into the build the builder comes and tells you that need to make a decision about what alternate material to use for the doors, the skirting's, the kitchen or bathroom, all you need to do without a PM is to spend time and source the materials yourself at a cost that you can bear.
Another thing you need will be fully annotated Structural Engineers Drawings, and finally you need a bills of Quantities compiled by a Quantity Surveyor so you can hand the bills Of Quantity to your preferred set of builders to get them to supply costs to you.
Very few if any large supposedly well planned construction projects go ahead without a hitch or two, some large and some small issues, but don't forget a hold up cost your money!
if you run with the Architects PM at a cost that is in excess of what you can fund and something goes wrong you have a better chance of a claim against the Architectural firm because they were offering a seamless service.
it as seems likely you run the PM with a friend and something goes wrong there is then a bun fight between all trying to apportion blame for the increase in costs, this is why PII [Professional Indemnity Insurance] is so costly to all Architects.
Only hint I have is if you want to try? to smooth out some future problems is to issue the bills Of Quantity to the Builders, then call a meeting where all the tendering builders, you your Architect and Quantity surveyor have a face to face meeting and any Builder un-sure as to what EXACT cost to include is thrashed out with all relevant persons and participants present, this way all builders are pricing against a consistent end.
Whatever you decide to do, I for one would be interested to know how things go for you?
ken