Consider a draftsman instead to modify stock plans.

The answer is, "it depends". It depends on where you live and what requirements your building authority may have for permits and drawings. Many times, if you need revisions for a plan to suit local codes and the site, you could have done the job cheaper by hiring the architect for an original design that was designed from the beginning to be compliant.

Also look at buying an existing home rather than paying the premium for something all new to you. Many times the pricing will be less, and you can still modify it to fit your needs.

What city are you building the house in? Architects are rarely required for residential construction.

Several comments. First, have you checked with your local authorities to determine that you do in fact need signed and sealed drawings to pull you permit? Check with your permit office and start right off the bat in developing a good working relationship with them. Each locale has different requirements. Some require signed and sealed drawings even for residential to pull a permit (NYC comes to mind). Some require just the structural to be signed and sealed, in which case you would want a local engineer familiar with the specific local requirements (codes) and conditions. Some locales don't require anything other than the plans, but you still need to know exactly what they are looking for in the submittal package (floor plan, electrical plan, mechanical plan, site plan, etc., etc.). Architects by the vary nature of being registered and licensed are prohibited from signing and sealing any drawings that they did not produce or directly supervised in the design and production of..........it's called "ghost sealing" and can end up costing the architect his license. That's not to say it's not done, but I would stay away from any architect who would do it.....whores will do just about anything for money.

If you don't need signed and sealed drawing, any competent designers (draftsman) can make modifications to an off the shelf design package. I know some design houses on line even offer design packages on CD's (dwg.file....cadd file) for an up charge for modifications. The starting point is your local permitting office.