Although bathroom suite complete sets fitting is well within the capabilities of any competent DIY person there's a number of different skills involved so many people prefer to reach for the phone and get professionals in. As well as plumbing there may be carpentry and joinery to be done, tiling and perhaps even building work if the bathroom is being significantly changed.

Bathroom Fitting Problems
The other problem with bathroom fitting is that quite often removing the existing bathroom suite will reveal a catalogue of horrors and suddenly the job becomes bigger than first realised. If an over-the-bath shower has been raining water down the side of a bath for years because the sealant isn't in good condition it's not unusual for the first sign of trouble to be the collapse of the ceiling below it.
And in extreme cases that might mean the replacement of joists and other supporting timberwork. This is a job that might not be technically difficult but it needs training and experience to know how to shore up a floor safely, and for this reason an untrained person might be better off steering clear of it.

Straightforward Replacement of Bathroom Fittings
But assuming the job is a little more straightforward and all that's required is the replacement of a bathroom suite and redecorating to a new style or design, then bathroom fitting is well within the reach of a DIY enthusiast. Most of the work will be reasonably easy as long as the stopcocks have been located. It might be worth investing in some specific plumbing spanners though. These are designed to work on out-of-reach and out-of-sight nuts, such as those securing pipework to taps on baths.
Where things will get tricky is if the new design requires relocation of the bathroom suite, and perhaps radiators too, rather than putting the new bathroom fittings in the same place as the old.

Use a Plumber for More Complicated Jobs
It might well be worth considering the services of a plumber in this situation as they will know the best way to extend and re-route the pipework to the new locations for the bathroom fittings. In fact a plumber may well point out that certain positions will be very difficult, if not impossible, as the fittings may not even work in the proposed new positions.
Financial Implications
For some people it may be a case of do-it-yourself or not at all. If money is tight then employing a professional may be out of the question; the choice is between DIY or not at all. If the decision is not that clear cut, people can work out how much they earn per hour then compare it with what the going rate for a professional such as a plumber.
If it's anything more than 80 percent of what a professional would be paid, then it might be worth taking that option. The reason it's 80 per cent as opposed to the same is because a professional who does bathroom fitting day in, day out will do the job a lot faster. For people how don’t actually enjoy the work anyway, employing a professional then becomes by far the better option.

Or is it Quality that Matters Most?
But no matter what the financial calculations reveal, there are some people who will do it themselves anyway, purely because they do not believe that anyone else will care as much about the fit and finish as they will.