Cleaning Nest Boxes - is this strictly necessary?

Hello - we're thinking of having an attic conversion on our house.  We currently have House Sparrows nesting under our eves and in the roof space.  This space will go when (if) we do the work.  We'd like to add some Sparrow nest boxes under the windows of the new roof space rooms (north facing).  Is this a good position? 

The second issue is that it would be very difficult to clean the boxes in this position.  Is it strictly necessary to clean boxes every autumn?  After all, we've never cleaned out the current nesting sites.

Is it simply better not to put up boxes if you can't gt to them to clean them?

Thanks in advance.