Building en-suite bathroom/shower in bedroom

Of course its possible and your correct in thinking it could potentially add value to your property. Weve done a similar project a couple of times previously and the end result is pretty impressive

ideally you should go and see the job, how can anyone say yes it’s possible without seeing where it’s going, are they floorboards or large moisture board flooring on the floor, where do the joists run ? is there access to soil stack pipe, will pipe work wastes have correct falls etc

Nothing it's impossible, the important thing it's a good planning and design to optimise at maximum the space, you can place the sanitary where it's the best for waste pipe allowance, and of course to match the budget what is offered. I've build new bathrooms in spaces which others claimed it's impossible.. so you have to adapt on site with all the possibilities you've got. Regards.

a qualified plumber can sort this out for you and give you advice.

It is to an extent dependent on plumbing, especially as you have a flat, key considerations are the 4" waste pipe, this might not be possible, but a macerator could be used, the downside is the noise these generate which some people don't like, another issue is ease of access to hot and cold supply pipes without the cost of lifting and replacing wood , laminate or tiled floors. In a nutshell just about anything is possible, it just costs money, and of course permission from your local council. A survey from a bathroom fitter is a must, hope this helps Carlo