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Martha Andrews FAIA
Andrews Architects, Inc.
Portland OR
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My advice, based on a practice focused on multi-family, is to calculate a basic fee that will cover the most likely set of variables and best guess of final design configuration; and keep the add-ons and adjustments that the client sees to a minimum. For our projects, we try not to get buried in a cascade of bookkeeping items that have to be tracked internally, documented to the client, and are a possible cause for dispute (and maybe not getting paid for).
It may be that we will calculate our base fee as you suggest (ea. unit type X multiplier for duplication), but the client would not see this, just the total. If we have done our homework enough to know upfront that there is an extensive planning or review process, we include this as an itemized line item as part of our fee proposal. And as the project design moves forward we seek to get client approval(s) so that if there are major plan variations at the client's request, we CAN bill this as additional service. The AIA standard contracts spell this out pretty well.
We don't recommend proposing to the client a low basic fee -- then doing a menu of add-ons -- because what the client will remember (and maybe only budget for) is the low base fee. Then when there are add-ons, even if quite legitimate from the architect's standpoint, the client feels like they've been a victim of bait & switch. This is a variation on the Owner's horror-story of getting eaten alive by Contractor's change orders.
In summary, we recommend keeping your basic fees and optional add-ons (if any) as simple as possible.
Martha Peck Andrews, FAIA
Andrews Architects, Inc.
Portland, Oregon