The important part to tell your boss (or HR, or whomever) isn't WHY you are doing it, but how you will be making it work for your projects/clients/etc. Frankly it isn't anyone's business why, and if they have a policy already they shouldn't be asking.

I was in the same situation, and simply went to 80% time when I realized I was unhappy.

I think they may be more likely to agree now, if they feel bad about not giving you the promotion.

Honestly, in this work climate, I wouldn't rock the boat. You are so lucky to have a job.

You can just tell them your husbands schedule changed even if it hasn't. It's not like they send an investigator to check.

I work part-time but I did it after having a baby, nearly 6 years ago. Be advised that you will certainly not get promoted now (or, at least, are unlikely to be). You do take a career hit when you do it, but for me it make me like my otherwise hum drum job. I have no advancement potential where I work because I have a different educational background than my peers. I think part-time is a great compromise if you know you're not going anywhere upward but you're not ready to leave. If I ever go back full-time, it would be for a different workplace.

If your husband's schedule really is about to change, it might make sense to wait -- but more to ensure that your new schedule will work with his. Otherwise, personally I don't see that you need a big life change to justify your request.

I don't have any experience with part-time but when I asked to telework half the week, it wasn't in response to any big personal event: I just said I that I'd like to discuss it with them, and then I set up an appointment and brought in my plan for how all the work would get done and how we would check in at intervals to make sure everyone was still happy with the arrangement. I did make the request shortly after not getting selected for a promotion, and I think they were more open to it because they wanted to "give" me something. But the plan for how all your current tasks will get done is the key.

I am thinking about asking to go reduced time at my work (where I work, there is a reduced time policy and several part-timers, so no issue there). There isn't a specific event that is necessarily causing this - for example, I am not just getting back from maternity leave, or my husband's work schedule didn't change. I did, however, recently find out that I wasn't getting promoted, and the reason that I didn't ask to go reduced time before was because I wanted to try for this promotion and didn't want my part time schedule to be an unspoken reason I didn't get promoted. Basically, now that I am not being promoted, I just want to have more time to see my kid (note, I am not at all worried that my lack of promotion means that my job is at risk - I am confident that I am a valued member of the team). My spouse thinks that it makes sense to tie my reduced time request with a specific event (for example, coming back from maternity leave (I'm not even pregnant again yet, so who knows when that will happen) or my husband's work schedule changes, which likely will happen in a few months) rather than asking now. Any one have any experience with this or advice?