So, you know exactly what needs to be done. Once, built, delivered and implemented the solution will solve all of the problems perfectly.
You present the solution and are promptly told, that will take too long, cost too much and take up too much space.
Unfortunately, this is the only approach that will work and there is no room in the budget so what can you do.
The solution is to remove some of the requirements. This is often a hard task as the requirements are only there because they are all required.
The way to decide which ones are removed from this phase is called MoSCoW.
This stands for must have, should have, could have and want to have. Each of the requirements should be categorised with one of these labels. This categorisation is a delicate activity as you must take into account not only current but future demands that are being developed in parallel.
Must have are requirements are fundamental and form the minimum usable requirements of the project without which the project is not worth doing.
Should have are almost as important as must have but short term workarounds exist to alleviate an immediate issue.
Could have requirements will not jeopardise current or near-future operations.
Want to have form ideal objectives but should be the first things considered to be omitted in the case of a resource overrun.
Appropriately applying these labels will allow you to more appropriately decide what should be omitted from the current stage, either being omitted or pushed to a later stage of development.