Whether or not all adults residing in an apartment are required to
sign the lease is left up to the individual property owner. Landlords
are not legally obligated to allow anyone who does not agree to their
requirements (as long as said requirements meet Fair Housing laws) to
live on their property.
If their rules state that you must sign the lease if you live on the
property, then you must sign the lease. If your SO signs the lease,
but you don't and live there anyway, both you *and* your SO can be
evicted on breach of contract grounds.
Some landlords don't require all adults to sign the lease, others do
*and* check the potential occupant's credit as well:
"A complete application is necessary from each adult (anyone 18 years
or older) who intends to reside at the property."
Assured Management, Inc.
By way of example, my current landlord did not require that I sign our
lease, but did require that my husband list my full name, as well as
the names of our children as occupants. The last landlord required a
full background check for both of us, and required that we both sign
""In leasing, there are three categories of people:
Residents (tenants), occupants, and guarantors:
Residents (tenants) are financially responsible AND have the right of
possession AND all other rights of a tenant (under landlord/tenant
laws of your state);
Permitted occupants (under your community's occupant rules) have
LIMITED right of possession, but they have no financial responsibility
-- they live there at the resident's grace (& under your policies);"
Consider the second paragraph in the above excerpt - occupants have
only limited right of possession and live there at the grace of the
resident (lessee). Your landlord's insistence that you sign the lease
too is for your own protection as well as that of the landlord.
For the sake of argument, I'm going to assume you and your SO are not
married. Suppose you have a falling out, and s/he kicks you out.
Suppose you have nowhere else to go, so you refuse to leave. If you're
not on the lease, the law is on his/her side, and s/he can legally
have you removed (evicted).
For housing laws specific to your state, you can check the following