Turning property into a 4 bed 2 bath

[Nonaski]

We’re looking at buying this property but just need some advice and if we are totally out with it all.
the first picture is the layout of upstairs as it is. It has a weirdly massive bathroom where all the red writing is is where the toilet, sink, bidet and bath currently sit.
we are hoping to turn it into a 4 bed with an ensiite in the bathroom room (making this become bedroom 4 with an en-suite) and adding a bathroom into bedroom 1. The question we’ve got is obviously the bathroom would not be on an outside wall but would be next to the pipework for the old toilet for the pipework.
also for this type of work we have read that we would need to get building control to come out sign off this type of work as it would be moving/adding waste pipes is this correct? Lastly does anyone know a rough cost to get the building control to come out?

thanks 🙂

You could have an issue with being able to add a loo to the new main bathroom. You need a straight run to an external wall, bends in a loo waste pipe are not a good plan.

Could you keep bed 1 as the master, and get a bathroom, ensuite and box room in the current bathroom?
Maybe with a dressing room and a bit of the ensuite coming into where you have sketched the new bathroom?

Edit: I've forgotten about the existence of doors. Don't think this will work. Sorry.

How's this? Sorry it's messy, was drawing on my phone with my fat fingers!
Green are doors, black walls, yellow new window, blue storage/wardrobes.

making the house 4 bedrooms is going to give you 3 quite small bedrooms. I would split the existing bathroom into a main bathroom & en-suite. If you need a fourth bedroom can you go up into the loft?

I think your plan looks fine. Get a bathroom expert in to quote and consider the toilet pipes, but it looks OK.

how are the ceiling heights? If you had to put the ensuite up 1 or 2 steps to have a void under the floor for the pipework, would that be possible?