Per Richmond Municipal Code Section 15.04.610.020, the following type of conversion ADUs shall be permitted on a lot with a proposed or existing single-family dwelling if they meet these standards:
An accessory dwelling unit within the existing space of an accessory structure may include an expansion of not more than 150 square feet beyond the same physical dimensions as the existing accessory structure to accommodate ingress and egress. The accessory dwelling unit shall have exterior access from the single-family home. The setbacks must be sufficient for fire and safety.
The City of Richmond may allow an expansion to square footage, though the addition to the ADU would have to comply with the City's ADU Ordinance.
If the expansion in floor area is not to accommodate an entrance or is over 150 square feet, the ADU could be allowed, but it would be subject to the local development and design standards, and the addition has to meet ADU setbacks. The City can allow for expansions beyond ingress/egress but those ADUs would be subject to local development and design standards, including the 4-foot rear/side setbacks and 5-foot setback from the primary dwelling. But, it's possible the 5-foot setback from the primary dwelling would have to be waived if that setback would prevent the development of an 800sf (max) detached ADU that meets exempt ADU standards.