Chrome extension that auto-ranks Zillow by cash flow — thoughts?

Question

Long-time lurker, first real post. I'm a casual investor with rentals in NJ and PA, and screening Zillow listings one-by-one was eating my evenings. So I built a Chrome extension for myself that:

- Pulls all data Zillow already shows on the map (price, taxes, est. rent, beds/baths)
- Runs cash flow + cash-on-cash on every visible listing automatically
- Sorts the map results so the top deals float to the top as you pan
- No clicking into each listing to do the math

Just got a triplex in NJ under contract that I found this way — would not have spotted it scrolling normally because it didn't look great on the thumbnail.

Before I share it more widely, I'd love feedback from people who actually do this for a living:

1. What assumptions do you use for vacancy, capex, and management when screening (not underwriting)?
2. Are there data points on Zillow you've found unreliable enough that they shouldn't be trusted in a first-pass filter?
3. Anything obvious I'm missing that would make a screening tool actually useful vs. just another calculator?

Happy to share more about how it works if there's interest. Mostly here to learn from people further down this road than me.

Andrii Lepesii · Answer

Really cool approach, Yiming — cash flow

ranking on the map view is something I haven't

seen done well yet.

I've been working on a complementary angle:

instead of cash flow math, my tool flags

pre-screening red flags before you even run

the numbers — price vs Zestimate gap, days on

market anomalies, and description keywords

that correlate with inspection issues.

The two approaches could work really well

together — yours for rental math, mine for

deal quality screening.

What stack did you build on? Happy to connect

and compare notes.

Natalie Schanne · Answer

@Yiming Jiang - if a property is in very poor condition, the zillow est value will be usually a fair - very good condition price. But very poor condition could be $100-150k of repairs, so keep an eye out for that.

The average property listed on zillow or MLS is vacant for 2-3 months before being rented. There’s time for fix up. Time for marketing. And then if someone sees it on 15th they usually sign for the 1st so that’s another 2 weeks of lost money. Plus the 1/2-1 months commission you pay the property management or realtor to rent it for you. If your rent is market rate, I’d say on average the tenant stays 2 years.

The best case is when you have a highly desirable unit in a highly desirable area with friendly tenants who will allow you to show it the last 60 days of their tenancy, in those cases you can sometimes have 0-1 days of vacancy because you have one move out, do a day’s worth of fix up and get in the new one in. New Jersey has some certificate of occupancy requirements so some townships request you pay for an inspection, give you some violations then schedule another inspection, which could take 3-4 weeks total depending on how busy they are to “approve” your rental for re-occupancy.