I'm planning to replace 50's steel framed windows on an 18th C cottage with
timber casements. On the rear elevation these are 2300 wide and 1200 high so
I have some choice over numbers of casements and glazing bars. Does anyone
know of any design software that would help sketch out the options quickly?

Second question, I know there is a rule requiring toughened glass for
windows with cills below 800 mm. Does that apply both sides? My outside cill
is only 300 mm from ground level. (Can't easily dig it out, although I would
like to, because I would have to re-lay all the drains).