Yes, we at MHIC Construction, Inc. have constructed modular units on existing residential residences. We are based in MD so our requirements will be different then for your location but I would imagine they are similar in their basic requirements.
Here are some things to keep in mind if you continue done this path. The architectural drawing your DPS (department of permitting services) will require will consist of two independent sets of plans. One set will be drawn from either an Architect or an Engineer. Using an Engineer will usually cost a little more but if you use an Architect the DPS may require an Engineer to certify any load calculations so you end up paying for both and it can also take more time using both.
The second set of drawing will come from the modular company and more directly from a third party independent Engineer company. They are required and hired to certify the unit(s) the modular building company will build for you correspond with all the code requirements your locations DPS require. This is because the units will come completed so the typical County building inspector will not be able to see the work that has been preformed.
The typical modular unit(s) is usually set directly on a foundation but this does not mean that they cannot be set on an existing structure. If they are set on an existing structure, this is where the Engineer certifying loads really comes into play.
Your builder will modify existing structures, fabricate needed foundations and finish any unfinished areas once the unit(s) is set. Be prepared to complete both gable wall, merry walls (these are the two walls where the modular units meet together), plumbing connects, electrical connection, HVAC unit (unless the modular unit(s) can be designed with this unit), stairs, handrail and some painting.
What is nice about modular construction is you can go from a hole in the ground to a finished building within the time it usually take a company to frame a typical stick framed building. These units can be set where they are already painted, carpeted with completed bathrooms and kitchens.
The best thing is to try and find a company that has done this work before or are a company that is competent in their business accomplishments.
I hope this has helped in your understanding to the world of modular contraction.
Bruce Wagner
MHIC Construction, Inc.