Well, my situation is similar to Feijoa mum only I am insane and also have DS1's girlfriend staying here with us from the UK.
Yep, we have a huge house - 4 bedrooms with a semi self contained flat/rumpus room downstairs. Ds1 & GF have downstairs. DD1 has her own room. DS2 & DD2 share but that is about to change as Ds2 will soon be in his own room and the girls will share (dd2 & dd3). DD3 is in with us. I wish we had an 8 sweater van OR that DS1 would get his license.
I do notice an expense as I have 2 in nappies, which I never really had before. Otherwise there has been zero impact on us. Dd just fit in like everyone else. I was so worried about #5 but it really is fine much less stressful than I envisioned it being. If DD2 & DS1 had behaviour changes, everything would be blissful