I am wondering if converting our loft into a playroom/toy storage would be cheaper than turning it into a bedroom?
We won't sell this house for a very long time so adding value isn't really an issue for us.
Our neighbours have fully converted their loft into a bedroom except they haven't put in a fixed staircase (odd) only a pull down ladder. But they have 4 x velux windows and built in storage up there. No planning permission as isn't required here but they didn't bother with building regs either.
Would we need to comply with building regs for a playroom?
Building regs are concerned with safe access, fire safety and escape routes, ventilation and light... So although you might not have to comply with building regs (I don't know) I reckon you'll probably want to, for a room where children will be playing a lot.
your neighbours have spent money which they won't get back, because a non-compliant conversion adds negative value to a house.
If you just want to board it (lofts are not designed to be strong enough to use as habitable rooms) and use it for light storage and a few small people, then you will probably not get much ceiling-cracking. It will be very cold in winter because it will be on the wrong side of the loft insulation. If you start spending serious money on insulation and lining it will be wasted unless you carry out a compliant conversion.
If you put heavy furniture or groups of heavy people up there, it will sag.