We had a year of major building work with a baby in the house. The builders arrived when baby was 6 months, and started with a new roof, followed by loft conversion, then rear extension. It actually wasn’t too bad. I’d definitely wait until the babies are past the newborn stage, having people in your space is tough and I couldn’t have done that in the immediate aftermath. The timescale for the extension was: broke ground in may (baby 10 months), broke through to house in mid June with limited/no kitchen for about 8 weeks, inside works continued for a further 3 months. Entire timescale in baby terms: 10-17 months, messiest/most disruption 11-14 months.
The project took longer because we kept our existing kitchen until the new one was fitted (in a different location). This was really important with a baby in the house but the kitchen itself was located in the building site so I didn’t have easy access to it during the day when on my own with baby. Once the new kitchen was installed it was actually fairly easy but we had builders turning the old kitchen space into a utility and shower room.
Honestly, I probably would do it again, it wasn’t so bad (even though I had significant PND and a traumatic birth and a difficult baby), and I conceived our second child just after the new kitchen was put it. The builder was the first person to know our news in the hope it might hurry him along! I wouldn’t plan to stay with your parents though, you actually need to be onsite most of the time to make sure they’ve not done anything silly. You need to be out of the house for really dusty periods, but that was fairly straightforward with baby groups, nct friends, trips to the zoo etc.
You need to agree a plan with the builder that enables you to stay in the house, so keeping some kind of kitchen etc. You need to accept that you will eat ready meals or eat out a fair amount, but if you get a chest freezer you could batch cook/get food from cook and you will be ok. I’d also recommend big kitchen disruption over summer because shoving something on the bbq with some salad is quite easy.
5 years on and the same builder is booked in after Christmas to remodel the kitchen in our new house!
TL;DR: you should definitely do it because if you need the space and have the money you’re just going to get annoyed at the house. You should wait until babies are not still in the newborn phase.