Sorry Starlight - have been away camping.
You need to go into as much detail as you can about your Ds's (or is it DD's?) needs and how they impact on you, your partner, any siblings, daily life etc. My friend advised me not to clear up the house too much(no problems there!), especially any damage DS2 had done.
We got ours for the following reasons:
DS2, while not 'challenging' in terms of behaviour, is judged to be so because of the severity of his delays. So I went into a lot of detail about that - and also about the amount of time we spend working with him at home.
DS1 (undergoing assessment for dyspraxia) needs more 1:1 time.
My mother is seriously ill, so cannot look after the DCs any more (she used to do a lot) - the SW also took into account the fact that I may have to go over there to support both my parents more as her illness progresses).
I didn't ask for a particular number of hours - but my SW said that you should always ask for more than you expect to get, so put down 12 (we got 8).
I was very honest, and also talked an awful lot! She probably recommended DPs just to shut me up Hope this helps - but I do think it depends (sadly) a huge amount on where you live and what kind of SW you get.