Having the existing plans is a considerable advantage. If they are properly detailed they will answer structural questions that only removing materials will possibly answer. In the absence of existing plans any area of the house affected by the remodel would have to be investigated and measured. For a remodel that affects the majority of the house this requires the whole house be investigated and measured which usually adds at least a day to the time and cost of the design. Obtaining existing plans is worth pursuing for these reasons.
Sources would be the professional who designed the home, the homebuilder or a subdivision approval committee. On older homes and subdivisions this usually proves fruitless. On newer homes if all else fails even a floor plan brochure can be helpful.
Public Information Acts provide for open records making it possible to view or obtain copies of plans on record if they were submitted for permitting to a city. Copies may be acquired of any plan that is not copyright protected. For those that are a release must be obtained by the person seeking the copies from the professional holding the copyright. If this can not be obtained those records can be viewed but copies will not be provided.
This varies but plans prior to the 70's will likely be recorded on microfilm that does not provide readable copies. Plans recorded from the 70's through 90's should provide readable copies. Plans from 2000 to present may be digitally recorded and provide quality copies. There will fees associated with copying and searches especially through logs on older plans but these are nominal.
While having the existing plans is a considerable advantage proper investigation and measuring will result in accurate 'as built' floor plans as a starting point so do not fret this as it is quite common and part of the due diligence.
If there is an addition involved building lines and utility easements must be considered. These are areas that the house or addition must not encroach into that are adjacent to the property line. A survey will have been provided at closing. This along with subdivision restrictions will show or define the building lines and easement(s) if any. There may also be maximum height restrictions per the restrictions or city code or ordinance. Usually existing homes in the neighborhood will give a glimpse into what is allowed.
To the fun part, the design. The scope of the remodel will be dictated by what the existing house will allow and the budget. Almost anything is possible though if you are willing to spend enough money. The primary goal however should be to accomplish what is desired in the most accommodating and least disruptive way to the existing house. This approach will result in the most value for the budget.
Sometimes this is a matter of being creative or accepting something that at first might offend your sensibilities based on what you are accustomed to. A kitchen that is too small might be sufficient or even generous just by taking in a large walk in pantry. This might seem unacceptable but consider the actual storage space since walking space is considerable. A highly organized floor to ceiling pantry cabinet might be more than sufficient. If the only other options are moving load bearing walls or an addition then the cost of what is gained must be weighed against being accustomed to a walk in pantry. If it is that critical then an important question has been answered and how important it is to you should drive the decision making. Prioritizing will be a critical component of the design process.
While a remodel may not offer the blank slate a new design does it does not have to be limiting. There are always creative ways to accomplish the goals.
To that end communication is the key. The design process is dependent on the homeowner to convey what they desire. It is the design professionals responsibility to ask the right questions and provide the homeowner with the pros and cons to come to the best decisions. Even the most creative design fails if the goals are not met due to a lack of communication.
This starts with defining the overall scope of the project and what is desired. Desired involves both wants and needs. Needs should be defined first these are the primary reasons for the remodel. Wants are secondary and may have to be prioritized to accommodate the budget or addressed in a creative fashion.
It is often helpful to know why something is needed. Sometimes in understanding the needs there are options or solutions available. A seldom used room could possibly be remodeled to accommodate a need much more cost effectively than adding on.
Specifics are critical. Even for a modest master bath there is much to be defined. And by today's standards modest means something entirely different than it once did. It may require 200 square feet to accommodate two sinks, a tub, shower, toilet compartment and single good sized walk in closet. His and hers closets are often the first feature that is desired that will add footage. On a luxury home there may be 200 square feet of master closets alone. With amenities like built in chests, dedicated shelves for shoes and purses, hanging area for belts and ties and mirrored dressing areas that is possible. I have even designed coffee bars with a bar sink in dressing areas. On the other end homeowners sometimes consider tubs unnecessary and wasteful often in favor of a larger shower. All of this needs to be discussed in detail to insure it is customized for your wants or needs.
Kitchens specifics are equally necessary. A very modest kitchen may only take 100 square feet. This will be limited to the basic necessities though and very likely will not satisfy today's market. On the other extreme a kitchen in a luxury
home with an island with seating may require 300 square feet. The main sink, island sink, dishwasher, refrigerator freezer, cook top with hood or downdraft, ovens, microwave and pantry are just the main features to be considered. Will the cabinets have a plant shelf or go to the ceiling. If there is a plant shelf will the top of the cabinets stagger in their height. Will some of the cabinets step out for definition. Cabinet doors versus drawers. Will there be glass in any of the cabinet doors for display. Does there need to be a desk area. Will there be pendant lights over the island or bartop.
The more fleshed out in every aspect before the start of the actual design the more likely the design will most successfully meet all the goals.
To that end do not be shy about being prepared with pictures and or notes if you like. Most of all allow yourself the time to fully comprehend and consider all that is necessary in the decision making process. Being excited to move forward can lead to rushing through the process. When someone says 'I wish I had considered' it just might have been with a little more time for discussion or review.
I hope this helps if you go forward and I wish you many years of enjoyment from a successful remodel!