How accurate the drawings of the existing building need to be really depends on how the particular architect works. Personally, I never fully trust existing condition drawings, even when I've personally measured the building with my laser distance finder, so I always work in some extra tolerances to account for the inevitable "busts" in the existing conditions. On the other hand, I know architects who will want to know if a room is slightly trapezoidal and design their alterations down to the 16th inch based on the not-quite-right angles.

Thank you for the reply. I used a laser distance finder myself. I'll likely keep my SketchUp file simple and communicate to the architect that there are some areas that are off an inch or two and they can decide how to handle it.

My guess is that the architect is going to redraw your idea model into a construction document model/file to generate the drawings- so even if its within an order of magnitude accurate, it should be helpful to facilitate the conversation. Good for you for taking initiative to start thinking through your spatial aspirations. 

I'd be interested to hear if the modeling effort is helping to reinforce and/or modifying your earliest/initial design assumptions.

I think the effort helps. I don't really have the ability to look at a space and just imagine what it could be, I need to "see" it. I suspect one of my ideas will look terrible once I model it.

It helps pick materials and colors as well. I've worked with Blender for a while so I can do my own archviz renders.

Just let your architect do his/her job.

If you want to help with ideas, gather images of spaces/things/materials you like; write down your goals for this project; be present & attentive, and actively consider what design advice is given. Be as clear and concise with your goals as you are able without defining the architecture. (For example: don't draw the kitchen. Instead, tell the architect what is important to you about how the kitchen needs to work for you. How do you use it? What don't you like about your current kitchen? Provide a list of appliance types that are essential [48" fridge, range (or cooktop), etc]. Talk about how essential it is to have 2 dishwashers; or that a knife drawer be there in lieu of a knife block on the counter) Make timely decisions when options are presented. Ask questions when you do not understand fully what is proposed. That is your role as client.

Re: the model...My guess is that whatever assumptions you make in determining the sizes of things for the sketchup model will need to be redone to the architect's standard, unless you are willing to affirm to the architect that your work is reliable (ie, any mistakes resulting from relying on your work are your responsibility). Does the architect even use Sketchup? Or, is it importable to other modeling softwares without needing to be rebuilt anyway? Do the model for yourself, if you must, but don't try to do the architects job for him/her.

[this post is not intended to be obnoxious, so i do hope it is not reading that way]

I appreciate the input, and understand completely about letting the architect do their job. I've thought about what you said and I think this exercise is really more for me and helping to rule some things out. One idea in particular.

And I can 100% say I want a knife drawer and not a block on the counter.