Two Storey Extension Costs/Process - @50sq metres

[Kazzabob]

We are looking to do an extension next year when our mortgage is up for review. We had a really rough sketch done - two storey extension at the back, and a room built over the garage - it comes out at 49.57m sq. Does anyone know what this might cost to build and fill? I'm budgeting around 100k but am really (REALLY!) hoping it will come in less than this. We live in Oxfordshire.

Also, could anyone advise on what you need to do regarding planning permission (is it needed) and who did you get to do your drawings?

Thanks!

Costs work out at ~ £1000/sq m, but might be higher depending on where you live.

I've currently budgeted at 1500 per sq metre and then the cost of refitting the kitchen etc on top - hence the 100k.