Three or four baths a week?

It's not in the en-suite!

The DSD has her own en-suite and is the only one who has one. Why don't people read?

It’s not £50 a month. It may be £20 a month. For 32 hours enjoyment - it’s not bad!

No, I take a short shower. I'm, sadly, not remotely in the typical MN earning bracket and I need to watch my spending. Baths are not my priority and £50 a month could be spent on things which matter more to me. It cannot be a total surprise to you that for some people water rates, gas and electricity are substantial expenses.

What will you do with the answer to your question? Surely it doesn't matter whether she loves a bath or whether she is being ungrateful. Knowing the answer to this will not change the situation a single bit.
I totally get the inconvenience and the expense. I am not sure I'd want to fork out £50 a month for one person to use the bathtub either. I can't even remember the last time I had a bath, it seems such a frivolous expense in the current climate... but that also isn't helpful information to you.
You two need to find a compromise. If it's money.. maybe she can contribute. If it is the fact that she occupies your bathroom four times a week, maybe you need to switch rooms around so that she doesn't have an ensuite which she doesn't use and you can use it instead. Maybe she can reduce the number of baths she has altogether or have them at a different time of the day. Any of these solutions are not dependent on the answer of whether she is ungrateful, though.

A few baths a year is very mean when she clearly gets a lot out of them. What about giving her something to displace the water if you’re really concerned about cost, a few large camping water containers (with water inside) sat in the bath?

Baths being frivolous! I take it you hose yourself down from the tap with a sponge scourer.

What will you do with the answer to your question? Surely it doesn't matter whether she loves a bath or whether she is being ungrateful. Knowing the answer to this will not change the situation a single bit.
I totally get the inconvenience and the expense. I am not sure I'd want to fork out £50 a month for one person to use the bathtub either. I can't even remember the last time I had a bath, it seems such a frivolous expense in the current climate... but that also isn't helpful information to you.
You two need to find a compromise. If it's money.. maybe she can contribute. If it is the fact that she occupies your bathroom four times a week, maybe you need to switch rooms around so that she doesn't have an ensuite which she doesn't use and you can use it instead. Maybe she can reduce the number of baths she has altogether or have them at a different time of the day. Any of these solutions are not dependent on the answer of whether she is ungrateful, though.

Just lock the door from the outside.
It's true if she's her own en suite and bath is basically part of your en suite, rather than the communal bathroom, then she shouldn't be hogging it. Or even using it without permission.

If it's in family bathroom then it's reasonable to use it but not for more than about 45 mins max. Unless she's disabled.

You actually suggest that OP offers to switch rooms with DSD so that the step daughter has full claim over the bath whilst they have the one with the shower? Are you actually being serious?

I may be a lone voice here, but I don't like DC having baths.
Mainly because we are on a water meter, and our water is heated up overnight on economy 7. This heats one tank of water.
A bath uses ¾ of the water in the tank, because DC have it full and they wont compromise on that.
So when I am running out of hot water by noon, I have to pay daytime rates to heat the top ⅓ of the tank, because the top up thermostat only heats the top ⅓.
Daytime rate only gives me the top ⅓ of a tank of hot water and costs more than 3 x the economy 7 rate!
So it costs as much to heat up that ⅓ per hour as it does to heat an entire tank per hour overnight.
When DC are having a bath every single day, as they are inclined to do sometimes, it costs a lot of money as well as the inconvenience of constantly running out of hot water for anything else.
Having said that, a couple of baths a week is fine.
We also had a shower with less than ideal pressure, but I found a fantastic relatively cheap shower head online that lived up to all of it's promises of increasing the pressure.

Yes same here on the book

I'd just ration her, assuming money is tight, and I'd tell her that's the reason why. Of course she'll understand (or if she's spoilt she won't but that's her dad's problem).

A bath is £3.50, a 4 per week is £14, so £56 per month. If you and your husband also had 4 baths a week, that's £168 per month on baths. You would have to calculate the cost of a shower and subtract this though to be fair.

If you can afford £168 (minus shower costs) per month, then let her carry on. But if this is tight then it's not fair that she gets to have all the baths. One bath a week per person is reasonable, and if you don't want your bath on any given week you could donate it to her!

When we redid our bathroom we got a lovely big deep tub with the intention of using it as a nice treat approx once every 2-3 months. DSD enjoys using it 3-4 times a week and complains that her ensuite shower isn't usable as the pressure in her shower is 'too low' (its fine- not a power shower but totally usable!). Am I unreasonable to feel this is completely ungrateful attitude and an ensuite is a massive privilege (only she has one in our house) she spends up to 2 hours bathing in our bath and I've worked out each bath costs £3.50 in gas and water (it's 300 litre capacity). When challenged on why she refuses to use her own bathroom she just shrugs/ scowls. Her ensuite is a shower room with a toilet (no bath) for reference.