A lot. Turntables with a

A lot. Turntables with a roundhouse take up a large area. Have you looked into engine servicing ( the job of the engine hostler) ? A properly set up roundhouse can be a layout in itself.

Six foot square

About six foot square should fit  the turntable and roundhouse. I used a three foot square corner to fit the smaller Walthers 90 foot turntable and smaller 9 stall roundhouse. I would have liked to model all 14 stalls of the prototype roundhouse, that is modeled on that part of the layout, but 9 stalls was all that would fit in the allowed space.

Think operation versus scenic

If its a small layout and you need the turntable to turn locos that's operation, but may not require a full roundhouse.  The wall of the roundhouse can simply be part of the scenic backdrop.  If on the other hand you need loco storage then you need the length of the turntable, plus the length of the longest engine that will fit the turntable, plus track clearance from stall one to stall two, plus the space for the front doors of the roundhouse, plus the space for the back wall of the roundhouse, plus . . . . . OK what was said by a preveious poster, LOT'S!  Russ

Yes, I am leaning that way.  

Yes, I am leaning that way.

I'm afraid, too much...

I truly want both (TT + roundhouse) but it seems my roundhouse may get partially tucked under steep slopes... Looking directly at it., I'd see 1/2 of a fully built roundhouse... looking over an elevated track (on embankment), part of the roundhouse roof would be concealed under landscaping of that embankment. 

Compromises.  (Sigh). 

Overall area reserved is about 3x5 feet.  Some people confine their entire operations to that!

Also, remember additional services (removing ash, fueling, sanding) are usually on tracks approaching the TT.