Why don't people don't like extensions?

There are a few reasons, but there's nothing concrete (boom, boom).

Some extensions are literally 3 walls and a roof stuck on to the side of the house. If the extension wasn't tied in to the original house correctly then there can be long term stability problems. However there can be long term stability problems with any part of a house, so it's not necessarily a good basis for valuing one part over the other.

Some extensions are uninsulated and are just literally built of lines of blocks. This can cause cold & damp problems. Then there are flat roofs which historically have a reputation for problems with sitting water.

Having said that, we built two extensions on two houses and they were well built, were insulated and had flat roofs that had no problems, so it's not like every extension has this issue. That's not going to stop people from assuming that all extensions are less valuable than the original house though.

My advice - consider things like insulation, cold, damp, etc . . . for *every* part of the house you are looking to buy and don't single out extensions specifically. If this means that you value a house more than someone who doesn't like the extension then you have an advantage over them in terms of bidding for and getting the house.

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Some extensions are not of as high a build quality as the house, so the value per sqm is adjusted accordingly. In extreme cases, the only thing that should be done with an extension is to demolish it.

I have seen extensions where the build quality is quite acceptable, but I don't see them as adding much to the value of the house. Typically they are sun-rooms or conservatories accessed from a living-room. The room to which they are attached loses light, and often ends up as primarily a passage to the new room. The net gain to the occupiers is very small.

A well-designed and well-built extension can add to the value of a house, but many extensions don't deliver much.

There's a huge difference between an architect designed extension and a rectangle box added the back of a house . A proper extension allows light to flow to the old room ( middle of the house) it akdo maximises the new and old space) often a box added on makes the middle of the house dark and dull. It also makes the middle of the house no more than a corridor and the extension offers no extra space

There's the problem that somebody else will have done the extension to their taste and "lock" you into something you don't like as much.

In terms of "value" or payback, once you go past the basic house, very few additions or improvements will pay for themselves when you sell it on if you're looking at it from that angle.