Replacing a bath in a small bathroom
Taking one bath out and putting another in sounds like the simplest job in a bathroom. In a small room it is usually the one that overruns, because three separate things have to line up at once and only one of them is the bath.
Measure the waste before you measure the bath
The length and width of the new bath matter less than where its waste sits. A bath waste has very little fall to play with, and if the new outlet lands even slightly away from the existing trap position, the pipe either gains a dip that holds water or has to be rerouted under the floor.
Lift a board at the tap end and look. A waste that runs straight into a stack a metre away gives you freedom. One that threads between joists at the far end of the room, at the limit of its fall already, means the new bath's outlet must sit within a few centimetres of the old one. That single measurement decides which baths are actually available to you.
Check the wall while it is open
A bath coming out is the only time you will see that wall. Look at the plaster behind the old one, press the bottom of the studwork if it is a partition, and look along the bottom course of tiles for staining.
Old baths leak slowly at the seal and the evidence collects where nobody can see it. If the plaster is soft, or there is blackness in the corner that wipes off and comes back wet, the moisture has been there a long time and the repair is bigger than the bathroom. Homeowners in that position have been sent towards firms handling bath replacement in Atlanta and equivalents nearer home, and it is money better spent than tiling over soft plaster and discovering it again in two years.
Support it properly
A steel or cast bath wants its own feet bedded on something solid, not on a floating floor. An acrylic bath wants a full timber cradle and, in a small room, a bearer screwed to the wall along its back edge. Fill it and stand in it before the final seal goes on: a bath that flexes under load will tear any sealant you put on it within a few months, and in a small room the flex is harder to spot because the panel hides it.
The order that avoids a second attempt
Rough the waste in, set and level the bath, fit the taps while you can still reach behind them, tile down to the rim rather than to the floor, and seal last of all with the bath filled.
Doing the tiling before the bath is set is the commonest reason a small bathroom ends up with a gap at one end that silicone is asked to bridge. Silicone will bridge perhaps five millimetres and look reasonable. Anything more wants a bath panel and an honest reassessment.
Where it stops being a weekend
Moving the waste more than a short distance, a soil connection, a soft floor near the bath or any smell of damp under the boards are all past DIY. Those are plumbing and building problems that happen to be in a bathroom, and the fitting is the easy part once they are sorted.