Hipped roof? 20mtrs will be its highest point then, which means that actually it's height in most parts will be much, much lower than this.
If you think about it, 20 mtrs is still lower that most of what Alton's coaster lineup currently reach. When you factor in clearance as well, I don't agree that's a great height for an indoor coaster at all. They can of course just launch it which means a coaster can't be ruled out, but I wouldn't say a coaster is a given like everyone else seems to think. Even if they did launch it, it's land mass isn't very big for a significant layout. So if it is an coaster, it would have to be one hell of a good layout with awesome effects or it'll end up being short and crap.
This is why I’m inclined to believe that it might be a family or family thrill indoor coaster rather than a proper hardcore thrill indoor coaster (e.g. Rock’n’Rollercoaster). The area this building takes up is large enough to accommodate a family-size indoor coaster (it’s larger than TWD at Thorpe, which is 2,500m2, and also similar in size to the indoor part of Arthur at Europa Park), and a 20m building height would allow for a good 15m coaster height if they put the highest point of the coaster at the highest point of the roof.
For some idea, the new DarKoaster at Busch Gardens Williamsburg is going in a building no bigger than this ride will inhabit, and that looks really cool, with a high speed switch track that will allow it to do 2 circuits while running multiple trains!
The thing I’d ask you is; if this were to be a regular dark ride, why would they need a 20m high, 3,000-4,000m2 building with such an atypical shape? Most dark ride buildings are little larger than 10m in height, have flat roofs and aren’t as large as this building (for some idea, Duel is 3,000m2 or a little under even when including its indoor queue, and DBGT is 2,306m2 and 10m tall. Both rides also have flat roofs).
