How to add a static secondary IP to a DHCP interface using netplan?

It's actually way easier, you just add dhcp4: true to your static configuration like this (and disable v6 if you don't need it):

network:
    version: 2
    ethernets:
        ens3:
            dhcp4: yes
            dhcp6: no
            addresses:
              - 10.0.0.250/24

Answered on Ask Ubuntu

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The solution was quite simple, just set a static IP address and enable DHCP. Basically you just have to add dhcp4: yes to your configuration.

This configuration gave me a primary static IP address and a secondary DHCP assigned IP address:

network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s3:
      dhcp4: yes
      dhcp6: no
      addresses: 
        - 10.1.2.15/24
      gateway4: 10.1.2.1
      nameservers:
        search:
          - example.com
        addresses: [10.1.2.10]

The result of ip address show enp0s3 gave me:

2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 08:00:27:ab:cd:ef brd ff:ff:ff:ff:ff:ff
    inet 10.1.2.15/24 brd 10.0.1.255 scope global enp0s3
       valid_lft forever preferred_lft forever
    inet 10.1.2.96/24 brd 10.0.1.255 scope global secondary dynamic enp0s3
       valid_lft 3224sec preferred_lft 3224sec
    inet6 fe80::a00:27ff:fe20:2c40/64 scope link 
       valid_lft forever preferred_lft forever

The address 10.1.2.96 is the secondary DHCP assigned address as indicated by the secondary dynamic keywords.