You're using wrong key :home in params. Should be
def create
home = Home.new(params[:full_street_address])
home.save && home.home_memberships.create(:user => current_user, :owner => true)
end
BTW, in the form you missed f and label (if needed)
<%= form_for(@home) do |f| %>
<%= f.label :full_street_address, "Home address" %>
<%= f.text_field_tag :full_street_address %>
<%= f.submit %>
<% end %>