To ask how you would respond with this?

[MiaMarshmallows]

A child of 9 sulking excessively whenever they do not get their own way? For example they will be told they cannot do something and they will sit there with their head down for ages and a moody expression or will walk off and sulk. Or they will be asked to do something but will have to be asked several times before they do it and then they will go back to their room to sulk as soon as they have completed a tiny part of what has been asked. Would you answer change if the child was also acting like this with caregivers outside the family?

She is very strong willed, always has to be right and gets in a sulk very quickly.
I ignore the sulking but the other stuff is hard to take as in having to repeat myself several times in order to get her to do what needs to be done.
It's my granddaughter and I find it tough because when I speak with DD about it, she will often go and hug her and will not also reprimand the behaviour or explain why it is unacceptable.

Give her basic choices so she feels she has some control

I think I'd challenge the sulking and ask them to rationally explain it. I'd ask them what outcome they expect from using it - get them to walk me through it step by step.
I'd ask if they see any adults doing it, and explain it's perceived as a childish behaviour. (That one usually worked with mine, for some reason being childish was a fate worse than death.)

Do you think positively of the child? I suspect the child knows you think negatively of her and is responding accordingly.

She is telling you something through behaving this way

She gets plenty of love and positive praise. I love her. Just not this behaviour.

My godson does this and he has just been diagnosed with autism. It is quite an extreme behaviour for a neurotypical child. Are they a particularly rigid thinker in general?

@Ahardyfool

You could try the ‘choices’ technique perhaps. So: “would you like to clear the plates now or after you’ve put away your books”

I think I've been doing choices wrong! Mine tend to be more 'You chose - you can set the table now and we'll all enjoy a nice meal together; or you can have a huff, you'll end up losing your screen time & you'll still be expected to set the table"

@LizzieSiddal

Yeah, or just set the bloody table 🙄

The OP has asked for advice. I’m sure she’s tried “Just set the bloody table”

Just trying it is a world away from the words in the post I quoted...

She's known as being super well behaved at school so I don't think autism as surely wouldn't be able to choose how she behaves between home and school? She is as I say, very strong-willed, confident, extremely so and likes to have things down her way and gets sulky when it is not. I do think DD treats her like too much of a mini adult rather than the 9 Yr old she is. She thinks she is one of us, an adult and she is clearly not.

She is telling you something through behaving this way

Oh for goodness sake!!! Some children (and adults) sulk, others don't. I was probably a sulker myself, I wasn't telling anyone something, I just didn't like not getting my own way!

when I speak with DD about it, she will often go and hug her and will not also reprimand the behaviour or explain why it is unacceptable.

^^ this is the issue right here. There’s no consistency with the two caregivers. Your daughter needs to back you up or you need to agree on how you manage sulking going forward.