Design Compromise

Bob is going to build the best tree house ever, it's going to have turrets, rope bridges, and a lift. He's already designed the whole thing in his head, now he just needs to get the materials...

Unfortunately for Bob, his ideas prove unrealistic. If he's lucky, he ends up with a basic tree house with a rope ladder.

This is a trope about initial ideas and grandiose plans than ultimately are unattainable in real life.

A good way of showing this is holding the plans in front of the camera and then lowering them to show the finished product.

This is present in the real world and fiction. One look at concept art for some computer games can have you asking, "Why isn't the finished game like that?" Lack of time, money and resources is usually the answer.

Could be seen as a specific form of What Could Have Been.

Examples of Design Compromise include:

Video Games

Rivet City from Fallout 3: Concept art and the finished product
Knights of the Old Republic II was supposed to be a truly epic followup and was originally planned to be much larger than the original, as evidenced by the massive amount of content Dummied Out of the final release.
Concept art for Epic Mickey reveals a deeply disturbing vision of a rotting Disney universe. The finished game is pretty obviously a Lighter and Softer compromise between Warren Spector's original vision and Disney's brand-enforcing creative police.

Literature

The soldier digging in The War of the Worlds is planning on keeping civilisation alive underground. He managed to dig a small tunnel.

Western Animation

On Arthur, this happens a couple of times. Once when they had to rebuild the treehouse, everybody had great plans but they ended up basically rebuilding it as it was before. And again when Grandma Thora gave Arthur and DW her attic to use as a clubhouse - Arthur wanted to make it into an adventure park and DW wanted to turn it into Mary Moocow-topia. Neither happened.
The subversion of this trope is the entire point of the show Phineas and Ferb.

Real Life

Happens all the time in construction projects, usually due to financial reasons. One example is the Currie building at the Royal Military College of Canada. The original plan was to build it as a replacement for the Mackenzie building. Construction began with part of the Currie building built against the Mackenzie building but the college ran out of funds to complete the project, so they left the Mackenzie building standing and joined the Mackenzie and Currie buildings together. The result is one larger building that looks somewhat disjointed, though the consistent architectural styles help to mitigate the effect somewhat.
The famous cathedral of Notre Dame was originally supposed to have a massive central spire rising far above the two existing towers, which were meant to be far higher, as well.