Lots of algorithms are at play when planning a new network, existing capacity being one of them ... so in effect, there may not be much left on the existing 'ring' that you mention, Chrys ??
We are chucking mega-money at the FTTP roll out and deploying new spines is all part of that process.
Our partners (TCS and Tech M) have the task of producing a 'Ghost Plan' that takes into account all premises and will then break that down into individual splitter nodes, each node feeding 120 customers (this figure can be upped by 1 or 2 if certain criteria is met). They also plan the HE availability, new equipment, stores ordering etc.
The problem with a GP is that it is, simply put, back of a fag packet planning. So once we have the GP blueprint, our surveyors hit the area en-masse producing a finalised plan of AGG - IJ - SN, plus any daisy-chaining of SN's that may occur.
This finalised data then gets put into a tracker and the magic starts.
We work on a 10-week glide in a lot of areas which consists of week 1 - survey of SN, week 2 & 3 creating job-pack and civils works estimates, weeks 4 -8 all on-site civils work completed, week 9 & 10 cabling, splicing, testing and commissioning.
Usually from commissioning to getting the return e-mail confirming CRFS (Customer Ready For Service), is a couple of days to add on as well. But, once you see the civls lads start digging, it should be aprrox 8 weeks later when you can place an order.
Of course, the above is 'in general' and there are always grey areas, especially in our rural (final 20%) builds.