Keeping this short and hence simple, planning policies vary between local authorities but based on a national framework. One of the things applying across the country is the differentiation between "settlement areas" and countryside, with respect to housing development. Cities, towns and some large villages have settlement boundaries around them. You can always see where that occurs, the housing stops abruptly along the road.
Everywhere else, including open countryside and many rural villages, are outside established settlements and bound by different policies. I'm assuming, because you say it is 80m to the nearest dwelling, that this property is outside declared settlement boundaries.
If that is the case then typical policies applying would be that you cannot increase the floor area of the dwelling by >50%. This figure applies from when originally built. This is to prevent overdevelopment of the countryside. This is a typical figure, but in a highly protected National Park it could be a lot less.
However, just because something is proposed as outside policy doesn't mean it will be rejected. Clever planning consultants with knowledge of what has been allowed before in similar circumstances can test the boundaries. The issue for council planners is that they often do not want an application to go to appeal, so they compromise.
Every case is unique, so I can't comment on your specific.
You can generally book an appointment with a local planner to discuss your ideas. It used to be free to householders but many now charge. They will give you General guidance on the applicable policy, but do not expect them to say how far above they might stretch.
You would be unwise to buy the house without planning consent as it would be a risk, if the extension is central to your needs.
Equally, having agreed consent on a small house in a desirable location may well increase the value the owner thinks it is worth.
The answers above mine are well-meaning but relate principally to rules INSIDE a settlement boundary.