Expansion of existing bathroom (wetroom)

The bathroom is a small wetroom with a sink, a toilet, a shower and space for a washing machine. There are two drains built into the floor. There are heating cables in the floor. There is an extractor fan in the ceiling. As the flat is a loft apartment, the ceiling is on a sloping angle. At the lowest side of the bathroom, behind the wall, is an open space in the building that is unused and currently inaccessible (except through a small inspection window). The proposal is to remove part of the low wall in the wetroom and build out the unused space as part of the bathroom to allow the installation of a bathtub. In the process, the washing machine and utilities to the sink and toilet would have to be moved. The toilet and sink themselves would not need to be moved. i believe that it would not be necessary to change the warming cables. The new area would have to be made as part of the wetroom with appropriate membranes etc. The floor of the expanded area may have to be built up to the existing level of the bathroom floor. The new area would need to be tiled to match the existing bathroom. The inspection window would need to be moved perhaps, and replaced with a proper velux roof window. The inspection window can be reached by climbing up the roof step ladder that is fixed to the roof from the balcony of the flat. Contact me and i will send pictures and a sketch showing the before and after layouts.