Moving in with boyfriend and his daughter-how do we do it right?

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My first post! Hope I am observing correct protocols.

My boyfriend of a few years and I want to move in together, with his daughter aged 9 who is with him half the week and every other weekend. He is a devoted dad. She's great, very bright, very talented, but runs rings around in a way that sometimes really wears him out (endless tantrums at breakfast about everything, a huge exhausting song and dance getting to school etc.) As I am not responsible for disciplining her, we get on fine and enjoy each other's company when we see each other, which isn't the whole time as I am currently not living with them. Obviously, when we are all under one roof I will need to be clear about some boundaries and she might not like that very much as she adjusts.
Their flat is a bit unsatisfactory for their current needs anyway (small, chilly, damp, mould...)
I am wondering if any of you have had any experience of orchestrating and presenting such a change. I don't have children and didn't spend much time around them until meeting my partner. I do want them though, and in the time I've been with him some of my friends have started having babies and I've got a bit more experience being around children.
I instinctively think it would be a mistake to solicit 'permission' from her-too much responsibility for a small person. But I want to present what is happening in the best and least anxiety-inducing way for her. She's recently started at a different school, and although she seems to be doing really well there, it's still a big change. The move is envisioned for Feb.
Any thoughts, experiences, suggestions appreciated.

You should check out a couple of other recent threads begun by women in your shoes

Oops, sorry, hit the post button too soon!
Meant to say - you should check out these threads in particular -
www.mumsnet.com/Talk/stepparenting/1312490-Move-in-together-or-keep-two-houses
and
www.mumsnet.com/Talk/stepparenting/1319072-Dont-know-what-to-do

Also, I'd strongly suggest that any changes in discipline, etc, should be well under way before you all move in together, and should be led/directed/instigated by your DP over the months before the move.

Thanks brdgrl-that's all v useful feedback and good idea about the introducing any changes before. :)

I've been living with DP and his son for over a year now, we had already had a one year relationship by the time I moved in. Dss didn't know I existed until 1 or 2 months before I moved in, I wouldn't have done it that way but Dp was too scared of his ex. Still, things went well, the kid's behaviour wasn't the best and tantrums were nearly an everyday thing, DP was not really doing much about it so it would be annoying sometimes because during the first months you can't do anything about it. I wouldn't advice you to start setting rules and boundaries right away or before you move in. You want the girl to be on your side, it'll be easier to change her behaviour if she sees you as a friend and more than obeying your orders, follows your example. It's been working great for us that way, kid behaves a lot better, school's been great lately, he respects people, has been showing to have developed manners and even helps around the house.
Just remember even though you've been together with your boyfriend for a while, you're in a way invading their home and you have to give the girl a little time to get used to the idea of you being there all the time.
If you really think introducing ground rules before is necessary, let your boyfriend and his daughter have a private talk, I would prefer it if she sees all those new rules as her dad's ideas rather than thinking you'll be the evil-fullofrules-stepmum. It also depends on her character and age, of course.
Spending weekends together and/or some of the days she's around also helps a lot, though I'm guessing it's been done and she is used to see you and her dad together.
That's just my opinion based on my experience. Hope it goes well!