Is it reasonable to ask other researchers to send me their previous grant applications? My university administration pushes all lecturers to apply to competitive grants from a national science fund. I am new to this and do not know how to start. So I thought to ask my previous advisors, or senior lecturers in my department, who work in related topics, and received such grants in previous years, to send me their past applications to the same fund. This may help me a lot to see how a successful application looks like.

However, I fear that this might be perceived as rudeness, since my application competes with theirs. The fund is very competitive, and if I send a competitive application, it reduces their chance to win a new grant. It's like asking a competitor in a code competition to show me their code..

Is it really considered rude to ask other researchers for their previous grant applications?
In my experience in both the US and UK this is quite common, and in fact encouraged. While technically, we are all in competition for funding, research works best when it is collaborative. There are so many nuances in a grant application that vary from across funding calls, that seeing successful (and even unsuccessful) applications can be hugely beneficial. they can give you an idea as to the scope of the project, how much it should build on previous work, and how much preliminary research should be done prior to applying.

I would hope current and past colleagues will share their funded proposals and would never hesitate to ask, and in fact would be mildly offended if they did not share them. Unfunded proposals are often helpful to see, but some colleagues are more hesitant to share unsuccessful ideas that they may still be trying to improve. In addition to asking to see proposals, you should be asking for feedback on your proposal as well as offering to read the proposal's of colleagues.

One aspect of a grant proposal that is difficult for new investigators (okay they are all difficult), is the budget. Many PIs will not share the budget of the proposal with others since they include salary information that many people consider private.