Do you need a new stadium, or do you just want one? The board are not going to spend hundreds of millions on a new stadium unless you're selling Old Trafford out every single week, and if I remember correctly it can be expanded to about 100,000 seats, right? What are your attendances like?
the lowest attendance was against Brighton - 93.000. I expanded the Old Trafford to 100.000 a few years ago but now that i have so much money i wanted to get a new stadium. I had 99.873 people for my match against Bayern.
I can see why it'd be frustrating, but unless you're selling out the stadium every week the board will be unwilling to invest so much money because they won't deem in necessary, I believe. I've had similar problems at different clubs in the past.
I don't usually manage small clubs so I usually never ever get a stadium.. currently I'm managing Barcelona (La Masia challenge - but I brought in Balanta, Pogba.. Digne in 1st season otherwise I'd be ********)
Shame though the Nou Camp is so big they just won't invest in a new stadium... I will keep begging.
Best thing to do is click on the premier league stats and you can sort by % average attendance, if this is high then you will have a better chance of them deciding to build a new stadium.