If you live in a semi or terrace .....

[mayblossominapril]

and an attached neighbours 3 year old woke up screaming and crying a few times per week between 11pm and 2am, would you be able to hear said child and how much would it bother you?

My neigbour can hear the kids at night (and during the day). During a particularly difficult period, they both came to see if I was ok. I’ll never forget their kindness. It must be annoying for them but they never complain and always tell me not to worry about it!

Depends on the house. In a new/newish build (anything built after 1950) most likely they'll be able to hear.
Older house, it depends on the construction and layout. We're in a terraced house built in early 1900s.
My baby is LOUD when she cries and the dog barks at any bird that flies past the window, every passer-by and every car. I worry about the noise and have asked both sets of neighbours how much they can hear from us. They all looked surprised and said they couldn't hear anything. We don't hear a peep from them either.

I would hear on the side attached to my bedroom. I would get annoyed if the crying was regularly going on for an hour. Nothing can be done about a child waking crying in the night, but if they can’t be quickly comforted then I think that the parent should take them downstairs where they are less likely to disturb the neighbours. An hour is a very long time to be kept awake.

Ah OP you are the parent of the screaming child. I'm glad you have decent neighbours right now. I think its good of you to consider implications upon future neighbours, but also I wouldn't search for a detached property for that reason alone. She's not going to be 3 forever, and hopefully at some point the night screaming will pass, just like everything else! My 2.5 year oldest night terrors were intense, and woke the baby who also started screaming, but that phase has ended for now. Hopefully your new neighbours will be empathetic... and you can always apologise and buy them ear plugs!