Any advice on buying a house with lots of previous extensions

[CovidCakeConundrum]

We are looking at buying a house that has previously had many little extensions (and some dodgy DIY).
Does anyone have any advice of what we should be checking? Its a bit of a mess with wires all over the place, running out of the building and back in again at places. Planning on getting a full survey and gas/electrical checks.
Using Google Street View i can see that the loft dormer extension has been done in the last 10 years. However I cannot see it or any of the other extensions on the council planning portal. Is this something I should be worried about?
Also I have noticed that the a chimney has been removed from downstairs but not in the room upstairs, is this OK?
Any advice from those more experienced is much appreciated!

I'd first be asking when they were done and if they were signed off by building control. It's fairly cheap to do at the time, it's inspected as the building work is going on, they check it's built to current standards. If they are fairly recent additions but no building control certificates I'd be wondering why and questioning whether it's worth paying for surveys and progressing. You can get indemnity insurance which covers you against enforcement action from the council if the work doesn't comply to building regulations if any of it wasn't signed off by building control if you did want to progress.

Get all the signoffs, get a decent structural survey and be ready for some weird building choices and a bit of effort in getting it how you like. Not necessarily something to worry about, just get the right checks done competently.

Loft conversions often fall under permitted development, AFAIK, and don't require specific planning permission.

Yes but they must comply with building regs, permission needed or not.
If they have removed a chimney breast below but not above, the weight needs to be supported, normally by an RSJ. This will also come under building regs.
Your lawyer will be aware of all this and will ask for appropriate documentation. As for indemnity insurance - it may protect you from the council but not from unsafe building work and if it needs remedial work you will have to pay.