The zoopla estimate is meaningless.
Offer what you think its worth now to you, it doesnt matter what the current owners paid for it, but dont be too surprised if they decline your offer.
You should be able to value it by looking at similar sized houses in the same vague area, If the same house( but minus the extension) generally sell for £155k in good condition, and this one is up for £210k, Is the extension worth £55k to you ?
Is there anything else about the house that justifies a higher price? A larger garden for example, or a south facing garden? You may not value a large south facing garden, in which case save some money and buy a house with a smaller north facing garden.
If everything about this house is almost identical to the others except for the extension, then It probably wouldnt cost £55k to build that extension judging by the description you’ve given. So the cheapest option could be to buy a house without the extension and get an extension built, but then you’ve got the stress and hassle of doing that, so what price do you put on not having to do that?