diff --git a/address/api/views.py b/address/api/views.py
index cf87d70..c915c4e 100644
--- a/address/api/views.py
+++ b/address/api/views.py
@@ -6,7 +6,8 @@
 from django.db.models import Q, QuerySet
 from drf_spectacular.utils import OpenApiParameter, extend_schema, extend_schema_view
 from rest_framework.exceptions import ParseError
-from rest_framework.viewsets import ReadOnlyModelViewSet
+from rest_framework.mixins import ListModelMixin
+from rest_framework.viewsets import GenericViewSet
 
 from address.models import Street
 
@@ -149,7 +150,7 @@
     OpenApiParameter(
         name="municipalitycode",
         location=OpenApiParameter.QUERY,
-        description='Municipality code, e.g. "91".',
+        description='Municipality code, e.g. "091".',
         required=False,
         type=str,
     ),
@@ -164,7 +165,7 @@
         + _municipality_parameters
     )
 )
-class AddressViewSet(ReadOnlyModelViewSet):
+class AddressViewSet(ListModelMixin, GenericViewSet):
     queryset = Address.objects.order_by("pk").prefetch_related(
         "street__translations",
         "postal_code_area__translations",
@@ -306,9 +307,8 @@ def _filter_by_location(self, addresses: QuerySet) -> QuerySet:
 
 @extend_schema_view(
     list=extend_schema(parameters=_area_parameters + _postal_code_parameters),
-    retrieve=extend_schema(parameters=_area_parameters),
 )
-class PostalCodeAreaViewSet(ReadOnlyModelViewSet):
+class PostalCodeAreaViewSet(ListModelMixin, GenericViewSet):
     queryset = PostalCodeArea.objects.order_by("pk").prefetch_related("translations")
     serializer_class = PostalCodeAreaSerializer
 
@@ -349,9 +349,31 @@ def _filter_by_post_office(self, areas: QuerySet) -> QuerySet:
 
 
 @extend_schema_view(
-    list=extend_schema(parameters=_area_parameters),
-    retrieve=extend_schema(parameters=_area_parameters),
+    list=extend_schema(parameters=_area_parameters + _municipality_parameters),
 )
-class MunicipalityViewSet(ReadOnlyModelViewSet):
+class MunicipalityViewSet(ListModelMixin, GenericViewSet):
     queryset = Municipality.objects.order_by("pk").prefetch_related("translations")
     serializer_class = MunicipalitySerializer
+
+    def get_queryset(self) -> QuerySet:
+        municipalities = self.queryset
+        municipalities = self._filter_by_municipality(municipalities)
+        municipalities = self._filter_by_municipality_code(municipalities)
+        return municipalities
+
+    def _filter_by_municipality(self, municipalities: QuerySet) -> QuerySet:
+        municipality = self.request.query_params.get("municipality")
+        if municipality is None:
+            return municipalities
+        municipality_ids = list(
+            Municipality.objects.filter(
+                translations__name__iexact=municipality
+            ).values_list("id", flat=True)
+        )
+        return municipalities.filter(id__in=municipality_ids)
+
+    def _filter_by_municipality_code(self, municipalities: QuerySet) -> QuerySet:
+        municipality_code = self.request.query_params.get("municipalitycode")
+        if municipality_code is None:
+            return municipalities
+        return municipalities.filter(code=municipality_code)
diff --git a/address/constants.py b/address/constants.py
index e2d3d67..c0b9ec7 100644
--- a/address/constants.py
+++ b/address/constants.py
@@ -1,62 +1,62 @@
 # Municipalities in the Uusimaa region: https://avoindata.fi/data/dataset/kuntaluettelo
 # The municipality names are not included in the Digiroad data, so this table is used to
-# map each municipality number to the Finnish and Swedish names.
+# map each municipality code (3-character string) to the Finnish and Swedish names.
 MUNICIPALITIES = {
     "uusimaa": {
-        18: ("Askola", "Askola"),
-        49: ("Espoo", "Esbo"),
-        78: ("Hanko", "Hangö"),
-        91: ("Helsinki", "Helsingfors"),
-        92: ("Vantaa", "Vanda"),
-        106: ("Hyvinkää", "Hyvinge"),
-        149: ("Inkoo", "Ingå"),
-        186: ("Järvenpää", "Träskända"),
-        224: ("Karkkila", "Högfors"),
-        235: ("Kauniainen", "Grankulla"),
-        245: ("Kerava", "Kervo"),
-        257: ("Kirkkonummi", "Kyrkslätt"),
-        407: ("Lapinjärvi", "Lappträsk"),
-        434: ("Loviisa", "Lovisa"),
-        444: ("Lohja", "Lojo"),
-        504: ("Myrskylä", "Mörskom"),
-        505: ("Mäntsälä", "Mäntsälä"),
-        543: ("Nurmijärvi", "Nurmijärvi"),
-        611: ("Pornainen", "Borgnäs"),
-        616: ("Pukkila", "Pukkila"),
-        638: ("Porvoo", "Borgå"),
-        710: ("Raasepori", "Raseborg"),
-        753: ("Sipoo", "Sibbo"),
-        755: ("Siuntio", "Sjundeå"),
-        858: ("Tuusula", "Tusby"),
-        927: ("Vihti", "Vichtis"),
+        "018": ("Askola", "Askola"),
+        "049": ("Espoo", "Esbo"),
+        "078": ("Hanko", "Hangö"),
+        "091": ("Helsinki", "Helsingfors"),
+        "092": ("Vantaa", "Vanda"),
+        "106": ("Hyvinkää", "Hyvinge"),
+        "149": ("Inkoo", "Ingå"),
+        "186": ("Järvenpää", "Träskända"),
+        "224": ("Karkkila", "Högfors"),
+        "235": ("Kauniainen", "Grankulla"),
+        "245": ("Kerava", "Kervo"),
+        "257": ("Kirkkonummi", "Kyrkslätt"),
+        "407": ("Lapinjärvi", "Lappträsk"),
+        "434": ("Loviisa", "Lovisa"),
+        "444": ("Lohja", "Lojo"),
+        "504": ("Myrskylä", "Mörskom"),
+        "505": ("Mäntsälä", "Mäntsälä"),
+        "543": ("Nurmijärvi", "Nurmijärvi"),
+        "611": ("Pornainen", "Borgnäs"),
+        "616": ("Pukkila", "Pukkila"),
+        "638": ("Porvoo", "Borgå"),
+        "710": ("Raasepori", "Raseborg"),
+        "753": ("Sipoo", "Sibbo"),
+        "755": ("Siuntio", "Sjundeå"),
+        "858": ("Tuusula", "Tusby"),
+        "927": ("Vihti", "Vichtis"),
     },
     "varsinais-suomi": {
-        19: ("Aura", "Aura"),
-        202: ("Kaarina", "S:t Karins"),
-        284: ("Koski Tl", "Koskis"),
-        304: ("Kustavi", "Gustavs"),
-        322: ("Kemiönsaari", "Kimitoön"),
-        400: ("Laitila", "Letala"),
-        423: ("Lieto", "Lundo"),
-        430: ("Loimaa", "Loimaa"),
-        445: ("Parainen", "Pargas"),
-        480: ("Marttila", "S:t Mårtens"),
-        481: ("Masku", "Masko"),
-        503: ("Mynämäki", "Virmo"),
-        529: ("Naantali", "Nådendal"),
-        538: ("Nousiainen", "Nousis"),
-        561: ("Oripää", "Oripää"),
-        577: ("Paimio", "Pemar"),
-        631: ("Pyhäranta", "Pyhäranta"),
-        636: ("Pöytyä", "Pöytis"),
-        680: ("Raisio", "Reso"),
-        704: ("Rusko", "Rusko"),
-        734: ("Salo", "Salo"),
-        738: ("Sauvo", "Sagu"),
-        761: ("Somero", "Somero"),
-        833: ("Taivassalo", "Tövsala"),
-        853: ("Turku", "Åbo"),
-        895: ("Uusikaupunki", "Nystad"),
-        918: ("Vehmaa", "Vemo"),
+        "019": ("Aura", "Aura"),
+        "202": ("Kaarina", "S:t Karins"),
+        "284": ("Koski Tl", "Koskis"),
+        "304": ("Kustavi", "Gustavs"),
+        "322": ("Kemiönsaari", "Kimitoön"),
+        "400": ("Laitila", "Letala"),
+        "423": ("Lieto", "Lundo"),
+        "430": ("Loimaa", "Loimaa"),
+        "445": ("Parainen", "Pargas"),
+        "480": ("Marttila", "S:t Mårtens"),
+        "481": ("Masku", "Masko"),
+        "503": ("Mynämäki", "Virmo"),
+        "529": ("Naantali", "Nådendal"),
+        "538": ("Nousiainen", "Nousis"),
+        "561": ("Oripää", "Oripää"),
+        "577": ("Paimio", "Pemar"),
+        "631": ("Pyhäranta", "Pyhäranta"),
+        "636": ("Pöytyä", "Pöytis"),
+        "680": ("Raisio", "Reso"),
+        "704": ("Rusko", "Rusko"),
+        "734": ("Salo", "Salo"),
+        "738": ("Sauvo", "Sagu"),
+        "761": ("Somero", "Somero"),
+        "833": ("Taivassalo", "Tövsala"),
+        "853": ("Turku", "Åbo"),
+        "895": ("Uusikaupunki", "Nystad"),
+        "918": ("Vehmaa", "Vemo"),
     },
 }
diff --git a/address/services/address_import.py b/address/services/address_import.py
index bea2592..4b4c2fd 100644
--- a/address/services/address_import.py
+++ b/address/services/address_import.py
@@ -72,21 +72,10 @@ def _build_addresses_from_feature(self, feature: Feature) -> list[Address]:
         if not self._has_required_fields(feature):
             return []
 
-        # Create the municipality and street if they don't exist yet
-        municipality_code = int(value_or_empty(feature, "KUNTAKOODI"))
-
-        # Validate that municipality code exists in the province's municipality list
-        if municipality_code not in MUNICIPALITIES[self.province]:
-            # Skip this feature - municipality code not defined for this province
+        municipality = self._get_municipality(feature)
+        if municipality is None:
             return []
 
-        municipality_fi, municipality_sv = MUNICIPALITIES[self.province][
-            municipality_code
-        ]
-        municipality = create_municipality(
-            municipality_code, municipality_fi, municipality_sv, municipality_fi
-        )
-
         street_name_fi = value_or_empty(feature, "TIENIMI_SU")
         street_name_sv = value_or_empty(feature, "TIENIMI_RU") or street_name_fi
         if not street_name_fi:
@@ -248,6 +237,24 @@ def _create_street(
         street.save()
         return street
 
+    def _get_municipality(self, feature: Feature) -> Municipality | None:
+        """Parse and validate municipality code from feature and return a
+        Municipality instance, or None if the code is missing or unknown.
+        """
+        raw = value_or_empty(feature, "KUNTAKOODI")
+        try:
+            municipality_code = str(int(raw)).zfill(3)
+        except ValueError:
+            return None
+        if municipality_code not in MUNICIPALITIES[self.province]:
+            return None
+        municipality_fi, municipality_sv = MUNICIPALITIES[self.province][
+            municipality_code
+        ]
+        return create_municipality(
+            municipality_code, municipality_fi, municipality_sv, municipality_fi
+        )
+
     def _has_required_fields(self, feature: Feature) -> bool:
         """Check whether the feature contains street name, first/last numbers
         and municipality code.
diff --git a/address/services/import_utils.py b/address/services/import_utils.py
index 58d560d..7d93d67 100644
--- a/address/services/import_utils.py
+++ b/address/services/import_utils.py
@@ -17,9 +17,16 @@ def value_or_empty(feature: Feature, key: str) -> str:
 
 @lru_cache(maxsize=None)  # noqa: B019
 def create_municipality(
-    code: int, municipality_fi: str, municipality_sv: str, municipality_en: str
+    code: str, municipality_fi: str, municipality_sv: str, municipality_en: str
 ) -> Municipality:
-    """Create a new municipality if it does not exist already, and return it."""
+    """Create a new municipality if it does not exist already, and return it.
+
+    Args:
+        code: 3-character municipality code (e.g., "091" for Helsinki)
+        municipality_fi: Finnish name
+        municipality_sv: Swedish name
+        municipality_en: English name
+    """
     municipality, _ = Municipality.objects.get_or_create(id=municipality_fi.lower())
     municipality.set_current_language("en")
     municipality.name = municipality_en
diff --git a/address/services/municipality_import.py b/address/services/municipality_import.py
index f2ab528..e2a0472 100644
--- a/address/services/municipality_import.py
+++ b/address/services/municipality_import.py
@@ -62,7 +62,7 @@ def import_municipalities(features: Iterable[Feature]) -> int:
     @staticmethod
     def _extract_and_validate_fields(
         feature: Feature,
-    ) -> tuple[int, str, str] | None:
+    ) -> tuple[str, str, str] | None:
         """Extract and validate required municipality fields from feature.
 
         Returns:
@@ -70,7 +70,7 @@ def _extract_and_validate_fields(
             None if any validation fails.
 
         Required fields:
-        - NATCODE: Municipality code (must be valid integer)
+        - NATCODE: Municipality code (must be valid integer, converted to 3-char string)
         - NAMEFIN or NAMESWE: Municipality name in Finnish or Swedish
         """
         nat_code = value_or_empty(feature, "NATCODE")
@@ -80,7 +80,7 @@ def _extract_and_validate_fields(
             logger.warning("Municipality feature missing NATCODE, skipping")
         else:
             try:
-                code = int(nat_code)
+                code = str(int(nat_code)).zfill(3)
             except ValueError:
                 logger.warning(f"Invalid NATCODE value '{nat_code}', skipping")
 
diff --git a/address/tests/factories.py b/address/tests/factories.py
index 21784ba..4d8b8a3 100644
--- a/address/tests/factories.py
+++ b/address/tests/factories.py
@@ -7,7 +7,7 @@
 class MunicipalityFactory(DjangoModelFactory):
     id = LazyAttribute(lambda o: o.name.lower())
     name = Faker("city", locale="fi_FI")
-    code = Faker("random_element", elements=("91", "92", "106"), locale="fi_FI")
+    code = Faker("random_element", elements=("091", "092", "106"), locale="fi_FI")
 
     @post_generation
     def post(self, *_, **__):
diff --git a/address/tests/test_address_import_service.py b/address/tests/test_address_import_service.py
index 52ac2b2..ca66cc2 100644
--- a/address/tests/test_address_import_service.py
+++ b/address/tests/test_address_import_service.py
@@ -11,7 +11,7 @@
 from ..tests.factories import AddressFactory, MunicipalityFactory, StreetFactory
 
 TEST_FEATURE_FIELDS = {
-    "KUNTAKOODI": 91,
+    "KUNTAKOODI": "091",
     "TIENIMI_SU": "Testikatu",
     "TIENIMI_RU": "Testgatan",
     "ENS_TALO_O": 1,
@@ -33,7 +33,7 @@
 @mark.django_db(transaction=True)
 def test_delete_address_data_removes_municipalities_streets_and_addresses():
     # Create municipality with ID matching one from uusimaa province
-    municipality = MunicipalityFactory(id="helsinki", code="91")
+    municipality = MunicipalityFactory(id="helsinki", code="091")
     street = StreetFactory(municipality=municipality)
     AddressFactory(municipality=municipality, street=street)
     AddressImporter(province=TEST_PROVINCE).delete_address_data()
@@ -71,7 +71,7 @@ def test_import_addresses_does_nothing_if_street_number_is_missing():
 
 @mark.django_db
 def test_import_addresses_creates_municipalities():
-    feature = _mock_feature({**TEST_FEATURE_FIELDS, "KUNTAKOODI": 49})
+    feature = _mock_feature({**TEST_FEATURE_FIELDS, "KUNTAKOODI": "049"})
     AddressImporter(TEST_PROVINCE).import_addresses([feature])
     assert Municipality.objects.translated(name="Espoo").count() == 1
 
@@ -79,7 +79,7 @@ def test_import_addresses_creates_municipalities():
 @mark.django_db
 def test_import_addresses_creates_streets():
     feature = _mock_feature(
-        {**TEST_FEATURE_FIELDS, "KUNTAKOODI": 149, "TIENIMI_SU": "CreationTest"}
+        {**TEST_FEATURE_FIELDS, "KUNTAKOODI": "149", "TIENIMI_SU": "CreationTest"}
     )
     AddressImporter(TEST_PROVINCE).import_addresses([feature])
     assert Street.objects.translated(name="CreationTest").count() == 1
@@ -87,7 +87,7 @@ def test_import_addresses_creates_streets():
 
 @mark.django_db
 def test_import_addresses_creates_addresses():
-    feature = _mock_feature({**TEST_FEATURE_FIELDS, "KUNTAKOODI": 186})
+    feature = _mock_feature({**TEST_FEATURE_FIELDS, "KUNTAKOODI": "186"})
     AddressImporter(TEST_PROVINCE).import_addresses([feature])
     street = Street.objects.get()
     expected_locations = [
diff --git a/address/tests/test_api_views.py b/address/tests/test_api_views.py
index a7b0284..cc3864d 100644
--- a/address/tests/test_api_views.py
+++ b/address/tests/test_api_views.py
@@ -4,7 +4,11 @@
 from pytest import mark
 from rest_framework.test import APIClient
 
-from ..api.serializers import AddressSerializer, PostalCodeAreaSerializer
+from ..api.serializers import (
+    AddressSerializer,
+    MunicipalitySerializer,
+    PostalCodeAreaSerializer,
+)
 from ..tests.factories import (
     AddressFactory,
     MunicipalityFactory,
@@ -105,7 +109,7 @@ def test_filter_addresses_by_municipality(api_client: APIClient):
 
 @mark.django_db
 def test_filter_addresses_by_municipality_code(api_client: APIClient):
-    municipality = MunicipalityFactory(code="91")
+    municipality = MunicipalityFactory(code="091")
     match = AddressFactory(municipality=municipality)
     serializer = AddressSerializer()
     response = api_client.get(
@@ -285,3 +289,39 @@ def test_filter_postal_area_codes_by_post_office(api_client: APIClient):
         "previous": None,
         "results": [serializer.to_representation(match)],
     }
+
+
+@mark.django_db
+def test_filter_municipalities_by_municipality_name(api_client: APIClient):
+    MunicipalityFactory(name="Vantaa", code="092")
+    match = MunicipalityFactory(name="Helsinki", code="091")
+    serializer = MunicipalitySerializer()
+    response = api_client.get(
+        reverse("address:municipality-list"),
+        {"municipality": match.name},
+    )
+    assert response.status_code == 200
+    assert response.data == {
+        "count": 1,
+        "next": None,
+        "previous": None,
+        "results": [serializer.to_representation(match)],
+    }
+
+
+@mark.django_db
+def test_filter_municipalities_by_municipality_code(api_client: APIClient):
+    MunicipalityFactory(name="Vantaa", code="092")
+    match = MunicipalityFactory(name="Helsinki", code="091")
+    serializer = MunicipalitySerializer()
+    response = api_client.get(
+        reverse("address:municipality-list"),
+        {"municipalitycode": match.code},
+    )
+    assert response.status_code == 200
+    assert response.data == {
+        "count": 1,
+        "next": None,
+        "previous": None,
+        "results": [serializer.to_representation(match)],
+    }
diff --git a/address/tests/test_import_postal_codes_command.py b/address/tests/test_import_postal_codes_command.py
index 3ced8c7..f37fe57 100644
--- a/address/tests/test_import_postal_codes_command.py
+++ b/address/tests/test_import_postal_codes_command.py
@@ -8,7 +8,7 @@
 
 @mark.django_db
 def test_import_postal_codes_updates_postal_codes_from_shapefile(paavo_shapefile):
-    municipality = MunicipalityFactory(code="91")
+    municipality = MunicipalityFactory(code="091")
     address = AddressFactory(
         location=Point(x=24.9428, y=60.1666, srid=settings.PROJECTION_SRID),
         municipality=municipality,
diff --git a/address/tests/test_postal_code_area_api.py b/address/tests/test_postal_code_area_api.py
index 10330d3..f39760c 100644
--- a/address/tests/test_postal_code_area_api.py
+++ b/address/tests/test_postal_code_area_api.py
@@ -28,21 +28,24 @@ def test_postal_code_area_api_includes_post_office_translations(api_client: APIC
 
     area.save()
 
-    url = reverse("address:postalcodearea-detail", kwargs={"pk": area.pk})
-    response = api_client.get(url)
+    url = reverse("address:postalcodearea-list")
+    response = api_client.get(url, {"postalcode": "00900"})
 
     assert response.status_code == 200
     data = response.json()
 
-    assert data["postal_code"] == "00900"
+    assert data["count"] == 1
+    result = data["results"][0]
 
-    assert data["post_office"] == {
+    assert result["postal_code"] == "00900"
+
+    assert result["post_office"] == {
         "fi": "HELSINKI",
         "sv": "HELSINGFORS",
         "en": "HELSINKI",
     }
 
-    assert data["name"] == {
+    assert result["name"] == {
         "fi": "Helsinki Keskusta",
         "sv": "Helsingfors Centrum",
         "en": "Helsinki Centre",
diff --git a/address/tests/test_postal_code_import_service.py b/address/tests/test_postal_code_import_service.py
index 13206da..eee07d6 100644
--- a/address/tests/test_postal_code_import_service.py
+++ b/address/tests/test_postal_code_import_service.py
@@ -16,7 +16,7 @@
 
 @mark.django_db(transaction=True)
 def test_import_postal_codes():
-    municipality = Municipality.objects.create(code=91, id="helsinki")
+    municipality = Municipality.objects.create(code="091", id="helsinki")
     address = AddressFactory(
         location=Point(x=24.9428, y=60.1666, srid=settings.PROJECTION_SRID),
         municipality=municipality,