Heat pump quotes vary more than almost any other trade, because the same nominal system can involve wildly different amounts of work depending on what already exists in the house.
The load calculation
A properly sized system is based on heat loss and heat gain for your specific house — insulation levels, window condition and area, orientation, air leakage, ceiling heights. Not floor area.
This matters financially as well as technically. A well-insulated house needs smaller equipment than a draughty one of identical size, and smaller equipment costs less. A contractor sizing from square footage will tend to oversize, which costs more upfront, cycles too frequently, wears out sooner and never runs at rated efficiency.
Ask to see the load calculation. A quote produced without one is an estimate dressed as a specification.
Existing ductwork
Reusing sound ducts is far cheaper than replacing them, and dramatically cheaper than installing ducts where none exist.
The complication in Washington is that a great deal of older ductwork runs through unconditioned crawl spaces and leaks badly. Installing efficient equipment onto leaking ducts wastes much of the benefit, so a good contractor will price sealing or replacement rather than ignoring it.
Where no ducts exist at all — common in older homes heated by baseboard or wall units — ductless mini-splits are usually the sensible route. That changes the cost structure entirely, since pricing becomes per-zone rather than per-system.
Electrical capacity
This is the most common hidden cost in Washington heat pump conversions. Older homes frequently have service panels sized for an era of gas heating and gas cooking, and a heat pump can exceed what they support.
A panel upgrade is a separate job with its own permit and inspection. It is not the HVAC contractor inventing work — it is a prerequisite. Have the panel assessed during quoting rather than discovering the limit on installation day.
If you are also considering an EV charger or an induction range, get the panel assessed for all of it at once. Sizing the upgrade twice is considerably more expensive than sizing it once.
Backup heat and climate zone
West of the Cascades, mild winters mean a heat pump rarely leaves its efficient range and backup requirements are modest. East of the Cascades, cold-climate specification and a considered backup strategy are part of the design, and dual fuel configurations pairing a heat pump with a gas furnace are common.
That adds cost, and it is the correct cost. A Spokane system specified to Seattle assumptions will disappoint in January.
Efficiency rating
Higher efficiency costs more upfront and less to run. Whether that pays back depends on how long you intend to stay and what you pay for energy. It is a genuine trade-off rather than an obvious upgrade, and a contractor who explains it that way is being straight with you.
Incentives
Utility and state programmes supporting heat pump conversion have existed in Washington and availability changes over time. A contractor working in this space regularly will know what is currently open in your utility district. It is worth asking before committing, because it can change the net cost meaningfully.