SB5033 – Limiting the property tax exemption for improvements to single-family dwellings to the construction of accessory dwelling units.
Prime Sponsor – Senator Kuderer (D; 48th District; Bellevue) (Co-Sponsors Saldaña, Das, Nguyen, and Claire Wilson – Ds)
Current status – Scheduled for a hearing in Ways and Means Thursday January 27th at 4:00 PM.
Next step would be – Scheduling a hearing.
Legislative tracking page for the bill.

Summary –
In the House 2022 –
Reintroduced in Ways and Means January 10th.

Current substitute –
The law currently provides a three year property tax exemption for any physical improvement to single-family dwellings on real property, including constructing an attached or detached ADU, “to the extent that the improvement represents 30% or less of the value of the original structure”. [I think that probably means that up to 30% of the cost is exempt, rather than that you can’t get the exemption if the improvement costs more than 30% of the value of the original structure.] The bill would limit the exemption to the construction of ADUs.

In the House 2021 –
Referred to Housing & Local Government Jan 13th; had a hearing there on the 21st; replaced by a substitute and referred to Ways and Means. Died there.