Concrete slab solar assessment Canberra

Hi
Im having trouble getting anyone to provide useful advice on going for an insulated slab North facing extension.
I had Lighthouse group do the energy assessment last year (which is now all redundant due to ACTPLA changes but still reasonable guidance), but my brief specifically included to advise whether a well insulated slab would work in the depths of Winter,, given my solar access is limited due to masive apartment block at NE and sun slides overr EW original roofline relatively early in the afternoon. On 20th June its 8.30am - 1pm (ish) I have the exact times written somewhere.
My issue is the best Lighthouse could offer was a verbal, very casual 'it should work'. Didnt fill me with confidence.
Builders Ive spoken to have varied between complete DGAF to 'just install inslab heating mate' - which could prove an expensive folly. I dont really want to install slab heating if I dont have to and vice versa if it will be chilly underfoot. Prepared to pay and install more solar to offset costs if necedsary.
So most simply -
given a well insulated polished slab with about 80% of tthe N facing wall being double glazed, good insulation etc etc whats the minimum sun hours required to have a good level of confidence my extension wont be a fridge underfoot during the coldest months in Canberra?
I realise theres a bunch of variables, maybe theres someone who's made this mistake and can talk from experience? Are there proper assessors who can do some calculations?

I have friends with a "similar' design that gets significant solar hours right through to dusk and theirs regularly gets too warrm over a few Sunny Winter days. Id much prefer to have that problem.

Any advice appreciated
Ramjet