Is "Plannable" an English (UK) word?

Is plannable, (e.g.: this task can be planned, it is plannable) an actual word in UK English?

It's one I see used quite often (mostly in business scenarios, both spoken and in emails) but haven't been able to find in on-line dictionaries and is always underlined by spell-checkers.

Research suggests that it's in fairly common usage although not in any dictionaries. I found some examples and interesting comments here: proz.com/forum/linguistics/…

Can't find it in any on-line dictionary, so probably not an "official" word (TrevorD's suggestion is probably the best grammatically correct usage for now).

However it is in fairly common usage, mostly in a business environment, so will likely be understood from context and possibly added to dictionaries over time.

From your reference it is clearly in use - and in the US as well as the UK.

Personally, if I needed to express that meaning, I think I would use can be planned for or an appropriate variation of that.