UI/UX in agile team vs technical limitation of design

I am a team leader with decent programming background. I learned basics of basics of UI design during my studies, however I am far from being experienced in this field. So please forgive me my ignorance and/or wrong understanding of how things works:)

I am struggling with designing best work flow for my team. Currently we have one sprint where UI/UX designer, who is working on the same feature as rest of the team.

However it seems that this flow is not efficient - planning phase take way to long (and by long I mean it is taking about 1.5-2 days to plan 1 week sprint), and i have feeling that technical talk of developer influences design.

I was thinking about moving to "do the UI/UX work one sprint earlier" approach, however I cannot resolve one issue: On one hand I would like to not influence creative process of UI/UX design by technical details, but on the other hand I am afraid that the designer will end up with things that are too costly to develop (i.e. we currently do not store X in our system, and adding that would add N weeks of additional work) and in the end will be forced to do the work again (=wasted effort).

• Should UI/UX know about tech details or will they disturb creative process to much?
• How do you handle that in your environments? Do you have some kind of meeting where UI/UX design discuss proposed solutions with developers?

Best Regards