When I was about 17 my dad had a bad car crash, smashed his pelvis and was unable to climb the stairs for a couple of months. That's the sort of experience you don't forget, and I also really want a separate study downstairs. Tweaking the design to give us a shower downstairs and make the study big enough to act as a guest room/bedroom if needed really didn't have very much of an impact on the design as a whole, and gives that confidence.
Roof-light is non-negotiable for us - it makes a big difference to the internal lighting/feel, and doesn't seem likely to cause overheating going by PHPP. What we're thinking of tweaking in the design is to use one of the triple-glazed rooflights which are compatible with the GSE system (Velux and Fakro do these). We then only lose one panel, compared to the 6 lost in the current layout. We're waiting until after permission is granted then apply to make the changes at the same time as we submit the detailed materials, etc.
NE facing roof is up for grabs - it gives us about 600 kWh/kWp, and doesn't need any changes to the inverter, etc. We've got quite a lot of generation (~10 MWh/year) from the SW facing roof and the NE direction doesn't give us very much in winter, so it's both relatively cheap and somewhat questionable how much benefit we would actually get.
Similarly with a battery - we would effectively be using summer exports to the grid to subsidise winter consumption, and given that we're cost limited then a battery is looking more like a retrofit a few years down the line. Plan is to have all the electrics up in the loft, along a partition wall behind the doors. That means changing over would just require swapping for a hybrid inverter and fitting a battery. It is up for grabs though - if budget allows I'm very tempted to go for a hybrid inverter to give mains backup and get myself a suitable battery off Alibaba once the dust has settled and have a go at fitting it.