Old Trafford can't accommodate one. It's an old stadium.
In truth the ground needs rebuilding: Knock it down, build over the car park, create an underground car park instead, and use the additional space to make it bigger and better. You could also potentially make the forecourt in front of the megastore smaller to make space for a bigger ground.
If we rebuilt it we could make Old Trafford truly state of the art. However, it will probably never happen because: 1) Money. The cost probably doesn't seem worth it to the Glazers. 2) Money & City. If we rebuilt we'd probably have to ground share with City for a season. Most of us would hate having to play at Wastelands. We would also have to pay rent for doing so, which the board would probably hate as much as the fans.
So there you go.
I'll just add to this comment a bit.
There has in fact been estimates done by consulting firms detailing the cost of both upgrading the stadium and its capacity versus simply building a new one. Unfortunately due to the public infrastructure immediately surrounding Old Trafford, the cost for doing so becomes astronomical. Expanding Old Trafford is so costly that it's not financially viable. It's not as much a question of what does the Glazers feel is worth it, as how realistic is this to begin with.
It's been deemed more feasible to simply build a new and bigger stadium on a new location.
However the Glazers choose not to do this since Manchester United play their games at Old Trafford. It's an institution you dont' "just move from". The fear is that the fans will completely turn against the owners and the club if they choose to abandon Old Trafford for a new stadium. Sure a new stadium would be nice, but football fans are passionate folk. We'd be talking about leaving home, and that's a though, though discussion.
The obvious second problem is where does the team play games. Ethiad? Considering the rivalry, that's not even going to enter the discussion.
So it's a bit of a mix: Too expensive to realistically consider + no immediate stadiums nearby with capacity. vs Moving the location vs Angry fans who don't want to move. That is why Old Trafford is getting maintenance upgrades yearly. OT was upgraded for £20 million this summer alone.
Let me lay this out in more detail so it makes sense why we are not expanding the stadium.
Behind the Bobby Charlton Stand there is a railway that would have to be moved for this expansion to occur. And on top of a railway shifting there is also the residents nearby that need to be rehoused. The stand would need to be built both up AND backwards for this to be structurally sound.
The bill for the upgrade project is estimated at £750 million, and could rise to £1 BILLION. - So essentially: If anyone wants to start a crowdfund for this project, I'll chip in the first £50.
I absolutely want OT to be upgraded, but it's just not feasible. it doesn't excuse a leaky roof by all means, but it's not as easy as pointing a finger and saying "cheapskate".
Oh, and there is new handicap sections being planned so overall capacity will go down a bit, to make place for our friends on wheels. By 2020.