What bleary said + that DIT may never be built so any gains you expect may not materialize.
With a mortgage of 660 K you have to consider risk. If house prices go back up and your income and that of your partners is unaffected by the economic climate you will may be ok. If they don't you will probably be in a very bad situation and very very stressed.
Can you sell your existing house? Have you tried?
Babys don't take up any more room that you have available. They don't even need a separate room immediatly. Have you tried rethinking your existing space. We added a loft ladder and floored the attic and it freed up loads of space in the house.
Size of house is only part of the deal, size of storage is also a major factor.
if youdon't have the followng I suggest you get them and see how you feel about your existing house:
1. Major clear out
2. Build in wardrobes (very affordable via cah ans carry)
3. Kitchen storage solutions (like more drawers, pull out larder etc, you can get this stuff retro fitted.
4. Floored attic
Good luck!