If they didn't get building regs, it could cause a resale headache later for you. You can apply for building regs in retrospect (they come out and inspect) or if you're not fussed about resale for whatever reason, you can go for an indemnity policy, but make absolutely sure you get a top range survey to ensure the work is sound.
Applying for retrospective building regs can be quite tricky as they will want to see steel work insulation etc all the things that are covered by plasterboard so you can end up with a fewbis to patch up afterwards.
We did a terraced house and to get the head height we had to drop all the first floor celings by two course of bricks. Also would highly recommend a dormer as the space gained is much better see photos.
My advice is if you can't be bothered or can't afford to do it properly wait till you can.
As previously said building regs arent there to cause hassle they are for your safety and to say the work is done to a satisfactory standard.
You may need a seperate ring from the fuse board, fire doors and casings through the house, mains smoke alarms the list goes on.
Get a good architect and you cant go wrong he will advise on everything some will even submit all the relevant paperwork for you to the council.
hope this helps