Good evening collective wisdom 
I am presently looking into building a single-storey granny annexe in our garden for my Mum, a independent 76 year old who is becoming sadly more infirm and won't be able to manage stairs for much longer. Her house is currently on the market so time is starting to tick a bit and I am starting to feel a bit overwhelmed by it all (especially as I am madly hormonal due to IVF meds at the moment).
So far we have had quotations from several "full service" annexe companies (the ones who do everything for you - planning permission, groundwork, build, fit-out, the lot) but these have proven problematic. Prices have changed, goalposts have moved, VAT has been added where previously there was none, calls have not been returned, etc. So it looks as if we are going to have to do it the hard and dirty way, i.e. by ourselves.
We are probably going to try and buy a log cabin (approx 5m x 9m). Does anyone have any recommendations of suppliers, or any words of warning? We will also need to apply for planning permission as our main property is Grade II and within a conservation area - not that the annexe will impact the main house or indeed the view, as the potential site is not overlooked.
If anyone has any hints/tips/experience to share I'd be really grateful - thanks in advance .