Can Design & Implementation sections be combined in a project report?
It seems you were given an assignment to design and build a system and then write a report to document your work. However, you decided not to do that assignment and instead you decided to build a system skipping the design step.

I would expect the person marking your report would see it as you did not complete the task as assigned. As JeffE's comment implies, there is a reason why you were given a task that included designing first then building.

I think you should ask your lecturer if what you have done is acceptable or if you must restart your assignment.