I live in a market town that had about 11,000 residents in 1990. Now the population is 15000, which is a huge increase in a small area. The infrastructure, typically, has not kept pace with the number of residents, and a mixture of the growing population and the system that allows children from other towns to use the schools (mostly rated 'Excellent') means that local children don't always get places, causing friction between 'incomers' and locals.
The trend towards working from home has meant that people from well outside the area have moved here because it is a lovely part of the country and the houses are cheaper than in the SE. They are still expensive for locals, however, so a lot of the new builds are bought by people with profits from smaller houses in more expensive areas, which can also cause tensions.
Modern houses are not my thing - I'd rather have good sized rooms and walls separating spaces than open-plan living areas and tiny bedrooms each with its own bathroom - but most house buyers must want that, and in any case there aren't enough older homes to go round. People seem to prefer detached houses crammed together in warrens than connected houses in wide streets with surrounding space and access to facilities.
As with other areas, most of the new houses are 4-5 bed boxes, clumped together on estates on the outskirts of town with no amenities. This means that everyone living there has to bring cars into the centre, clogging car parks and polluting the air. There seems to be no thought to it. The town centre (where I live) has developed over hundreds of years, so there is nowhere to put estates, so instead they are on ex farmland, and on spaces where there used to be facilities such as hospitals which have been lost to centralised 'hubs'. The town has lost a lot of its character and sense of self, and if things carry on in this way we will find ourselves joined up with neighbouring towns in all directions. The estates have definitely changed the character of the town, as they do all over the UK.
It is a shame in many ways, but all houses were new once, and people have to live somewhere. Demographic shifts and other shifts in social behaviour, such as divorce, young people marrying/settling down later and becoming less likely to live with parents until they do so mean that there are more households than there used to be, and more houses are needed as a result. Nobody wants them in their backyard, but where are they supposed to go?
I would say there are both, the latter probably reinforced by the former.