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Flatpack are pre-packaged furniture built with the Construction skill, and are made at the Workbench in the Workshop of a player-owned house. Flatpacks allow players to construct the furniture inside their player-owned house without the necessary level required to construct the furniture. An example would be a level 42 builder adding a Costume room to his or her house, and then buying a level 50 toy box with a flatpack purchased from a higher-level builder. The level 42 builder may then take the flatpack toy box into his or her house in building mode, and use the flatpack on the toy box hotspot and the box is built, even though the house owner is not level 50. No experience is received when using a flatpack on a building hotspot.
Flatpack items can be constructed with building mode either on or off. A hammer and a saw are needed in your inventory to build flatpack items, and they are always built un-noted. Each flatpack takes 8 game ticks (4.8 seconds) to create. Creating flatpacks still requires the same amount of materials needed to construct the furniture at the hotspot. The advantage of making flatpack items is that there is no need to enter building mode or remove any furniture from a hotspot.
While flatpacks are tradeable, there is not much traffic in flatpack items, and they do not gain much money at the general store.
Even though pieces of furniture from the study room cannot be made into a flatpack, the game cache reveals a flatpackable version of them. This may be the result of Jagex deciding that these items should not be made flatpackable for unknown reasons, but did not remove the code for the items before launch.
Upon the release of the Grand Exchange, players were able to submit buy offers for these items, but as they cannot be made in the workshop's workbench, the offers were never completed.
There's a number of flatpacks that exist as objects in the game but you can't actually create them, because they were created when Construction was being developed as a skill way back in... 2005? 6? The flatpacks that exist were scrapped but the objects weren't deleted, so they made it into the live game... we could make them flatpackable, but there's no real benefit to it.