Thanks, Glasnost. 
The things that worry me about this census have all been mentioned before, but just to sum up what I don't like about it:
It's being run by a dodgy US company that also sells arms = the info is not secure.
I don't trust the reasons given for why we need so many questions. (I've never minded filling it in before). They say it's for planning services but if they need to know how many children there are, they can go to the registry office. If they need to find out how many hospitals we need, they can look at the waiting lists. Even if they did find out that we need more schools or hospitals, it's not like anyone has any money (they say) to build new ones anyway. And even if they find out that your whole family is disabled, they are not going to send in an army of care workers.
Nothing is going to change as a result of this survey, except that all our personal information is going to be on one giant database that could be sold to the highest bidder or get 'lost'.
Questions like number of rooms etc can be useful for sociological research etc, but the info can be found more easily elsewhere.
So why are they spending so much money on it? I heard/read somewhere that it would cost less to go ahead with it than not to, but I can't see how that is the case.
Clearly one of the main reasons for some of the questions is to find out how many ethnic minorities, especially Muslims, are living in the UK. That could be harmless or even useful info in the right hands, but it could also be extremely harmful in the wrong hands.
And I agree with Glasnost that we have to question things and not let the authorities bully us.
It's their own fault if we don't trust the government. I really really want to, because I'd like to think they have our best interests at heart - as I always have done before. But my confidence has been severely shaken by the lies this lot have told, from tuition fees to selling off our precious NHS.