Timescale for building a new school and when will distances change for intake

[cb2ndtimer]

Our 1st choice school announced 3 weeks ago they have had approval from government for new school to be built.
It is a 2 form intake infants with attached preschool. Funding is already in place as school had been budgeting for this for last few years.
Just wondering if anyone had any rough idea of how long these things take - mainly under or over 12 months?
Also, when do you think the catchment distances will alter? 1st day of new school being open or any earlier?
We are 1km from current school but 700m from where new location is. LEA uses route taken to measure distance rather than as the crow flies. Hopefully this includes footpaths as road route would add another 150m at least.

Dd's school opened almost exactly a year (20 days over) from the day the very first machinery went onto the land. The school amalgamated three schools one infant, one junior and a primary.It takes 350 pupils to give you an idea of size. HTH

Thanks for your help.
The school will be the same class size as the current school (2 preschool and 6 KS1 classes) as far as I am aware, so I imagine the current children will just move there automatically.

The old school is in a built up area with no playing field or room for expansion. It will be closing completely and being sold for housing. The new site is just waste ground at the moment so hopefully will be easy/quick to build on.
I asked about distances because, as I understand it, if a child moves into an area and is nearer to the school they move higher up the waiting list. So if a new list was created for the new site there is a (slim) chance we would be higher up the list.
The footpaths I refered to could be classed as alleyways but they do have street lights. Tried looking on the LEA website (Derbyshire) to see any more detail with no luck.

Dd's school is a Derbyshire school too so the timescale should be a good guide. No trouble with places there though because the school they built had about 70 excess places although now full as word got out that the new school HT was going to make a huge difference. The school opened April 18 months later they let 5 into ks1 on appeal but the school was full. Maybe there will be extra places at the new build too.