diff --git a/changelog.d/15613.doc b/changelog.d/15613.doc
new file mode 100644
index 000000000000..15cd983ace45
--- /dev/null
+++ b/changelog.d/15613.doc
@@ -0,0 +1 @@
+Flake.nix: warn users that at least 3.75GB of space is needed.
diff --git a/changelog.d/15613.feature b/changelog.d/15613.feature
new file mode 100644
index 000000000000..11924da602cb
--- /dev/null
+++ b/changelog.d/15613.feature
@@ -0,0 +1 @@
+Flake.nix: `devenv up` now automatically starts synapse, configured to use redis and postgresql.
diff --git a/flake.nix b/flake.nix
index 7351571e61de..0df7f43f2c9b 100644
--- a/flake.nix
+++ b/flake.nix
@@ -1,19 +1,21 @@
-# A nix flake that sets up a complete Synapse development environment. Dependencies
+# A Nix flake that sets up a complete Synapse development environment. Dependencies
 # for the SyTest (https://github.com/matrix-org/sytest) and Complement
 # (https://github.com/matrix-org/complement) Matrix homeserver test suites are also
 # installed automatically.
 #
-# You must have already installed nix (https://nixos.org) on your system to use this.
-# nix can be installed on Linux or MacOS; NixOS is not required. Windows is not
-# directly supported, but nix can be installed inside of WSL2 or even Docker
+# You must have already installed Nix (https://nixos.org) on your system to use this.
+# Nix can be installed on Linux or MacOS; NixOS is not required. Windows is not
+# directly supported, but Nix can be installed inside of WSL2 or even Docker
 # containers. Please refer to https://nixos.org/download for details.
 #
 # You must also enable support for flakes in Nix. See the following for how to
 # do so permanently: https://nixos.wiki/wiki/Flakes#Enable_flakes
 #
+# Be warned: you'll need over 3.75 GB of free space to download all the dependencies.
+#
 # Usage:
 #
-# With nix installed, navigate to the directory containing this flake and run
+# With Nix installed, navigate to the directory containing this flake and run
 # `nix develop --impure`. The `--impure` is necessary in order to store state
 # locally from "services", such as PostgreSQL and Redis.
 #
@@ -66,7 +68,7 @@
         let
           pkgs = nixpkgs.legacyPackages.${system};
         in {
-          # Everything is configured via devenv - a nix module for creating declarative
+          # Everything is configured via devenv - a Nix module for creating declarative
           # developer environments. See https://devenv.sh/reference/options/ for a list
           # of all possible options.
           default = devenv.lib.mkShell {
@@ -153,11 +155,35 @@
                 # Redis is needed in order to run Synapse in worker mode.
                 services.redis.enable = true;
 
+                # We are now ready to start Synapse.
+                
+                process.before = ''
+                  python -m synapse.app.homeserver -c homeserver.yaml --generate-config --server-name=synapse.dev --report-stats=no
+                  mkdir -p homeserver-config-overrides.d
+                  cat > homeserver-config-overrides.d/database.yaml << EOF
+                  ## Do not edit this file. This file is generated by flake.nix
+                  database:
+                    name: psycopg2
+                    args:
+                      user: synapse_user
+                      database: synapse
+                      host: $PGHOST
+                      cp_min: 5
+                      cp_max: 10
+                  EOF
+                  cat > homeserver-config-overrides.d/redis.yaml << EOF
+                  ## Do not edit this file. This file is generated by flake.nix
+                  redis:
+                    enabled: true
+                  EOF
+                '';
+                processes.synapse.exec = "poetry run python -m synapse.app.homeserver -c homeserver.yaml --config-directory homeserver-config-overrides.d";
+
                 # Define the perl modules we require to run SyTest.
                 #
                 # This list was compiled by cross-referencing https://metacpan.org/
                 # with the modules defined in './cpanfile' and then finding the
-                # corresponding nix packages on https://search.nixos.org/packages.
+                # corresponding Nix packages on https://search.nixos.org/packages.
                 #
                 # This was done until `./install-deps.pl --dryrun` produced no output.
                 env.PERL5LIB = "${with pkgs.perl536Packages; makePerlPath [