We did this, OP. We had a two storey 6 metre x 4 metre extension at the back , and extended slightly at the front at the same time. We also took the opportunity to replace all the circa 1980 Artex ceilings and install a new lighting scheme and new radiators in all rooms, because they had all gone rusty. This meant there was work going on throughout the house and extension.
The work took six months. Our builders were building a village hall at the same time as they were working on our project.
DH and I slept in our caravan on the drive for six months, including one winter. This was OK unless we had forgotten to turn the caravan heater on one hour before going to bed. We had no bedroom in the house until the builders knocked through into the extension. The children slept in the house, as at first their bedrooms were unaffected. At the time they were aged 12 and 15. They had no parent in the house overnight for six months, but we were just outside.
It is all about the timing, too. DD's old bedroom was converted into the family bathroom, so obviously her new bedroom needed to be built beforehand so she would have somewhere to sleep ( although there would've been room in the caravan ). DS had to move into the spare room for a while so his room could have new ceiling, and radiator. We got through it fine, but if you have very young children there may be too many hazards during the works ( e.g. missing areas of floor with narrow pathway among exposed floor joists etc)
To sum up, I think it may take nearer to six months, as your DH says. You could still live in the house, but that depends on you all being flexible and keeping a great sense of humour. You won't believe the mess, disruption and DUST ! Oh the dust, everywhere. It will be worth it when it's done. I wish you a smooth time, no hitches and Good Luck.