Going part-time at current employer to fund start-up

Honestly, only you will know how your boss will react.

Are there other employees working part time? Maybe you can approach the subject from a different angle.

When you do approach it, make sure you approach them with their interests in mind. Offer to work full time hours every other sprint or so. Just remember, don't just ask part time, part time in most states is anything under 35 hours. Also, as part time, you're likely to loose any benefits (i.e. health insurance, 401k, etc.).

I think, before you approach the idea of part time at work, you should approach the idea of cutting a little bit of leisure our of your week. What are you doing all weekend? Why not take one day completely off, and work for a few hours after work each day, and devote one of your off days from your full time job to maybe a 10 hour day on the startup?

What type of life have you created for yourself that requires x amount of dollars/pounds/euros/whatever? Is there anyway to cut household costs? Maybe cut out cable and only have internet + netflix? Maybe cut out a home phone if you have one?

Or maybe, rather than moving to part time, ask to tellecommute one or two days a week? That will cut out whatever commute time you have, and save you some gas money, which you can in turn use to cut out your expenses and how much you require each month.

But, honestly in my opinion, if you're wanting your startup to succeed, I don't think you should go into it requiring a certain amount to live on because, from the sounds of it, you're used to living on a software dev's salary (what? ~$60k/year?). There will come a time where you will need to devote full time hours to your startup, but you will NOT be bringing in that much money to take home that salary.

I think you should reevaluate what's happening at home before you reevaluate what's going on at work. You're just as likely to be let go (no matter how important to a project you are/think you are) when asking to get part time as you are to actually get it. It's a risk that honestly doesn't sound like it will work with you.

If your startup makes it or not is a matter of guessing.

But I can tell from own experience of half-time startup work. I have a customer for whom I work 20 hours. My plan was to work the other 20 hours for my startup. The rest of the time I want to use for overhead, marketing, other startup related work and so on.

This is the theory. The fact is that sometimes my customer needs special attention. Some feature need hurry. Sometimes one needs to fix a bug quickly. Or we have a meeting with customers customer which are outside of my 20h range.

So reality is often 30h customer, 15h overhead and maybe 5 or 10 hours startup work. On other occassions i do manage to get out 20 hours for my startup, but I cannot count on it.

My advise to you:

Make a clear timeframe you work for your boss and when not. For example, say Monday and Thursday. Do not say something like 4 hours a day, because you'll not get done in 4 hours. Of course other options do the trick. Do not say 20 hours a week. Becaues if you do them on the beginning of a week most likely is your boss is expecting some other work to be done later the week.

Another question you need to answer:

how much time will it take you to develop your startup? I mean, in reality. Are you really succeeding with for example 20 hours of development time? Please don't forget the time you need for marketing, invoicing, relationship buidling and so.

If you manage I consider this a good plan. Personally even when my startup develops slowly, I am not afraid to fail in any way, my flat and food is paid. On some days i am just to tired to work on the startup - then i can sleep with the knowledge that I have no existencial crisis waiting for me when I wake up. After all my life is pretty easy and interesting.

About whether it is a common scenario, I would say no. Most IT people I know work-full time for a company or full-time as freelances. The only exceptions are maternity-related.

If it is a good or bad idea depends on the options. Options are:

a) Full-time job with leisure time for your startup (you have discarded it)

b) Full-time startup (you have discarded it)

c) Part-time job and part-time startup (I think we have a winner)

The first question I would ask myself is "Will my boss/company take this as an offense (because of thinking you are going to compete with them, because they will see this as looking down on them...)?".

If you think you know your answer, act accordingly.