NC as embarrassed 
I’ve been looking for a house for a while and missed out on a couple as I was too slow/bid too low
Saw a house a few weeks ago, on at 485 and offered 480 straight away (that day) - there were lots of viewings and I just acted impulsively! I now realise lots of viewings did not translate into lots of offers
Now having checked sold house prices I realise I’ve offered over by quite a bit ... ‘identical’ 2 bed houses on same street have sold between 415/several at 425/one at 450k but with a much larger plot - some of the prices are this year others I last 2-3 years. It’s the most expensive 2bed I the area by quite some way some I’m worried I’m making a mistake p. This is a new area/further out of London for me and I had not realised the difference in price
This house has a small extension which I’ve calculated would have cost around 20k to build (it’s 2x7m so quite small really), which adds value but it’s about half the garden so now very little garden (which may also make it hard to sell)
I like the property: seems good condition, like the street/location, I haven’t seen other houses I like better but feel very silly and worried I’d be selling at a loss if I move in eg 3-5 years . I’ve been advised my mortgage company will do a valuation which I won’t see but it’s 60pc LTV so not sure what outcome would be
Is there a decent way for me to renegotiate the price? Eg get a surveyors valuation ? Vendors are moving into rented, so I wouldn’t be messing up a chain, I’d like to renegotiate and buy the property but at a price more reflective of what there’s on street swelling at
I realise now I think I should have offered eg 460 perhaps... my offer was made 2 weeks ago & accepted a week later
Thanks in advance for any advice on what to do