But same kind of work (programming), and company can possibly use it for their business (you don't know, even if you think you do) or using even similar code snippets in both programs (and that happens, even only in non-business logic parts of the code doing unimportant stuff) would unfortunately lead to a situation with possibility of conflict. After all, they pay you, to program "things that can be useful to them", so that's your job. Does that mean you can sell them this specific program, just because they haven't yet ordered you to write it in your work time? What do you want to do if they order you at work to write something like you have written in your free time? Being sulky? Writing a "totally different" program from your free time program just because you want to waste their time and money? Bringing your home-grown program which violates all company quality standards to work? You cannot win either way. As said before, the time when you did it, does not matter. (Except of course: before the start and after the end of the employment, you are free (from this employer), but as long as you are emloyed, it does not matter if it weekend.) It is all complicated, because it is not just some stupid and easy to follow rules. Law is not what one side thinks is right, but often times a reconciliation of interests, and it gets messy.
Ask yourself: could a reasonable third party, without knowing your or your employers intentions or even technical details, draw a clear line between your free time work and the stuff you are paid to work on, only by looking at your free time project and the kind of work you and your colleages do in the office? If the only product of your employer is monitoring software and you write the stereotypical Tetris clone: yeah, that case might be easy. Real companies and useful free time projects? Not so much.
Also to train you to think like a reasonable third party: think of a bricklayer laying bricks in his "free time". (Sounds a bit odd, doesn't it?) When would this be acceptable, when wouldn't it? Not so easy. Of course you have to give something up for getting paid.
I would suggest not being a salary slave, but that would be hypocritical of me, because I am.
Get paid enough, save enough, make a clean cut, start project later.
Have other hobbies, go out in the sun, meet some friends, life is too short.
As others wrote: the programming is the easy part of a business, you can easily solve that after you lost your chains.
The line I draw: if it is only for my amusement and noone else can access the work, I will do it for myself. (Strictly speaking, this may also be a relevant work, but that's the line I draw. Some may say: daring, some may say: cowardly. Probably right. Both of them.)
Getting an exception for works within your area of work, for which you do not have to to transfer rights into contract supplement, should be possible at reasonable employers, but check before you start creating the work, don't let them think your work as employee will suffer, and if is commercial, a side-business or similar: that's another independent problem to take care of (no competition, maximum work time as required by law may be applicable, and so on). (Contract supplements are done all the time for more trivial stuff, no big deal. The world changes, and so do contracts.) A small employer might even say: if your work is not in this and this business field, we don't care. For bigger ones it will be the other way around: you will have to be specific on what you want to do and some legal department will check business interests, IP problems, if it is far enough away from your departments work, and so on. This might as well limit the work you can do in the company, so check your career plans.
Check if you need to program at all. The ready solution might be available as open source already. These days many employers also have open source policies, giving you some additional free space for your programming projects while employed, see if you can use that to your advantage. If you are good at project management / people skills: let others do the work.
So let's all stop bitching about the lawyers (a lawyer would have written a more understandable and concise text, but would have charged you more) and the law: the world is complicated and full of conflicting interests, don't expect the business world to work like your hobby projects with your friends. Don't be sad, grow up.