I hate this phrase because nobody ever defines what it means and almost nobody has a coherent view about what it would take for such a thing to exist. Yes there's a precise federal definition of it - related to local median incomes - but that isn't what most people who use the term mean. It's a term that means whatever people want it to mean!
All good people agree that people should be able to afford places to live (even if they can't afford anything). I don't object to that. But the "affordable housing" conversation is quite often directed at new construction, which is the weirdest place to focus on affordability. Building new construction is expensive and acquiring the land to build it on in high rent areas is also really expensive. It's the most expensive way to think about providing "affordable housing." Also, new construction has to face contemporary neighborhood concerns and contemporary land use regulations (without arguing these are good or bad they also make things more expensive!). Construction codes (safey, etc.) get ratcheted up regularly and while, again, this does not make them bad it makes new construction more expensive.
Locally the conversation tends to go something like this: developer proposes something (There aren't a lot of big plots in Philly, so most developments aren't massive. We aren't talking about razing neighborhoods or even blocks for things). The neighborhood group objects. Often people say they want more "affordable housing." Also, they want more parking. Also, they want single family homes (rowhouses, so attached single family, but still).
The thing is, granite countertops just don't cost much money relative to the whole. All new construction is "luxury" as you will notice if you read your local real estate listings. Not because they have golden toilets (or granite counter tops), but just because they're new.
The only way to make market rate housing affordable - and I'm zeroing out developer profits here - is to build smaller units and with less land/unit (no parking). Sure you can skip the granite counter tops, too, but that doesn't actually save much.
If I ran the zoo I'd build massive amounts of public housing along the British "council housing" model. But I don't.