Looks more like the city misrepresented that it was possible to change the zoning back to multi family after already changing it once. Take it up with whomever Firman is. But the city can change their minds on anything. Especially if it had already undergone the conversion of multi family to single family. Single family to multi family is a very difficult to achieve option almost anywhere outside CA.

When it comes to zoning, get everything in writing directly from the zoning administrator, signed, witnessed, with a thumb and index finger prints in their blood.

(can you tell I have had a bad experience with a zoning administrator?)

Even if his comment was written in bell clear English, Firman was misunderstood, misconstrued misread and misinterpreted. Speak to an experienced real estate lawyer to see if there is a remedy.

Anything related to zoning get it in writing, and even then it is not just one person decision. And zoning board words are not final until submitted your plans to the city, and they had chances to review them. One year later you can have different people on the zoning board and they will tell you totally opposite.

Isn't going to an architect about a zoning question sort of like asking your electician what to do about your dripping faucet?

This is a legal rather than a design issue. Before buying a property you should ask a lawyer to review all restrictions. Even an interpretation from the zoning enforcement officer is not adequate protection.

You are repeating your mistake by asking designers and homeowners for legal advice.

My question is who is responsible here, and do we have grounds to sue to architect? Thinking about getting a zoning attorney

First, yes you absolutely should get an attorney if you are interested in some recovery. However, you really need to have something to recover.

From your initial description you have a solid promissory estoppel action against the architect. The architect is pretty obviously a person who could be reasonably relied on to have interacted with the zoning commission, an architect should be reasonably aware that you would rely on them. They made a statement, you relied on it, they were wrong. So you can attempt to recover damages incurred because of that reliance. However, promissory estoppels are generally not the strongest cases.

Also, there are a few things anyone should understand before they go too far. There really needs to be some damages. If you spent $500,000 on this deal and sell (or could sell) the land for $600,000 then you are going to struggle to recover. You could still argue that other suitable properties have appreciated more than $100k in that time, but that starts to get pretty flimsy.

In the American system you have to pay your attorney fees even if you win. You can ask a judge to award you attorney fees, but winning attorney fees is rarer than people imagine. In my experience, attorney fees are awarded more often in cases where someone should have known better than to drag this into court. So keep in mind that your recovery may not even cover your attorney fees for trying to recover.

Finally, have you considered a conversation with your architect about this? Most people are good and reasonable but guarded. You might get better results by meeting them in the middle rather than starting in your corner. A credit to future design services may be a win-win for both of you.

Good luck

Did you do due diligence prior to closing to understand if the old multi-family was non-conforming, requiring the failed convert to single family?

Who is Firmin?

Zoning is generally applied to districts, not individual construction projects, unless an application for special use or exception has been submitted and approved by the jurisdiction.

Have you contacted the architect to ask for an explanation? Jurisdictions can and often do change zoning districts after public notice and hearings.

From the notes you posted it appears the architect contacted someone and got verification of an existing 2-unit zoning. Who is that contact? That seems like the starting point.

If it had been someone outside of the architect's firm, we probaby wouldn't be discussing the matter.

It might be Firmin Senga of Senga Architects.

Then if that is the architectural team, a good assumption the project is in the city itself. Have you first contacted your alderman, and/or given them any "political donations?"

Three years after receiving formal written Committee of Adjustment approval for variances from zoning, the Buildings Department in our fair burgh withheld approval based on--you got it--the design not meeting current zoning. Two months and many emails to and fro and we're still waiting approval. (They had zero objections to the architectural and engineering designs.)

Good luck!