Dear [client's name/s]
PROJECT NAME
It has been 6 months/12 months* since we last spoke/corresponded* so this is just a quick note to check if you have any questions we might be able to answer on the operation or maintenance of the building, or if you have any comments – objective or subjective – on how you are finding living in/working in/using* it.
We use all the feedback we get from our clients on previous projects to guide our work on future projects, with the aim of continuously improving our work and delivering value to our clients, so any thoughts you can share will be very much appreciated.
And of course if you have any other projects under consideration, please do bear us in mind!
I’m pleased to say we have recently [add note on recent project completions, planning wins, awards, new members of staff, etc].
I look forward to hearing from you.
Kind regards,
[your name] 

* Delete as applicable