diff --git a/Dan.Common/Models/CustomSubjectRequirement.cs b/Dan.Common/Models/CustomSubjectRequirement.cs
new file mode 100644
index 0000000..b08fbaf
--- /dev/null
+++ b/Dan.Common/Models/CustomSubjectRequirement.cs
@@ -0,0 +1,21 @@
+namespace Dan.Common.Models;
+
+/// <summary>
+/// Requirement for subjects that differ from organisation number or Norwegian SSN
+/// </summary>
+public class CustomSubjectRequirement : Requirement
+{
+    /// <summary>
+    /// Regex used to validate custom subject. Defaults to \w+ for any letters and digits
+    /// </summary>
+    [DataMember(Name = "subjectRegex")]
+    [Required]
+    public string SubjectRegex { get; set; } = @"\w+";
+    
+    /// <summary>
+    /// Describes the regex in clear text
+    /// </summary>
+    [DataMember(Name = "subjectRegexDescription")]
+    [Required]
+    public string SubjectRegexDescription { get; set; } = "Any string";
+}
\ No newline at end of file
diff --git a/Dan.Core.UnitTest/AuthorizationRequestValidatorServiceTest.cs b/Dan.Core.UnitTest/Services/AuthorizationRequestValidatorServiceTest.cs
similarity index 95%
rename from Dan.Core.UnitTest/AuthorizationRequestValidatorServiceTest.cs
rename to Dan.Core.UnitTest/Services/AuthorizationRequestValidatorServiceTest.cs
index 6c758fa..df08997 100644
--- a/Dan.Core.UnitTest/AuthorizationRequestValidatorServiceTest.cs
+++ b/Dan.Core.UnitTest/Services/AuthorizationRequestValidatorServiceTest.cs
@@ -1,4 +1,5 @@
 using System.Diagnostics.CodeAnalysis;
+using AwesomeAssertions;
 using Dan.Common.Enums;
 using Dan.Common.Models;
 using Dan.Core.Exceptions;
@@ -9,7 +10,7 @@
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
 
-namespace Dan.Core.UnitTest
+namespace Dan.Core.UnitTest.Services
 {
     [TestClass]
     [ExcludeFromCodeCoverage]
@@ -140,7 +141,43 @@ await Assert.ThrowsAsync<InvalidSubjectException>(async () =>
                 await arvs.Validate(GetAuthorizationRequest(subject: "123456789"));
             });
         }
-           
+        
+        [TestMethod]
+        [DataRow("03065001488")]
+        [DataRow("12345678")]
+        public async Task ValidateTest_CustomSubjectRequirement_ShouldSetParty(string subject)
+        {
+            // Setup
+            var ae = GetAvailableEvidenceCodes().Take(1).ToList();
+            ae.First().AuthorizationRequirements =
+            [
+                new CustomSubjectRequirement
+                {
+                    SubjectRegex = @"^\d{1,8}$", // digits only 1-8 characters long,
+                    SubjectRegexDescription = "Description",
+                    RequirementType = "CustomSubjectRequirement"
+                }
+            ];
+
+            A.CallTo(() => _mockAvailableEvidenceCodesService.GetAvailableEvidenceCodes(A<bool>._))
+                .Returns(Task.FromResult(ae));
+            
+            var arvs = new AuthorizationRequestValidatorService(
+                _loggerFactory,
+                _mockEntityRegistryService,
+                _mockAvailableEvidenceCodesService,
+                _mockRequirementValidatorService,
+                _mockRequestContextService);
+
+            var request = GetAuthorizationRequest(subject: subject);
+            
+            // Act
+            var action = async () => await arvs.Validate(request);
+            
+            // Assert
+            await action.Should().NotThrowAsync();
+            request.SubjectParty.Id.Should().Be(subject);
+        }
 
         [TestMethod]
         public async Task ValidateTestFailureWithInvalidRequestorOrgNo()
diff --git a/Dan.Core.UnitTest/RequirementValidationServiceTest.cs b/Dan.Core.UnitTest/Services/RequirementValidationServiceTest.cs
similarity index 88%
rename from Dan.Core.UnitTest/RequirementValidationServiceTest.cs
rename to Dan.Core.UnitTest/Services/RequirementValidationServiceTest.cs
index 7384a16..ce9dfb5 100644
--- a/Dan.Core.UnitTest/RequirementValidationServiceTest.cs
+++ b/Dan.Core.UnitTest/Services/RequirementValidationServiceTest.cs
@@ -1,4 +1,5 @@
-using Dan.Common.Enums;
+using AwesomeAssertions;
+using Dan.Common.Enums;
 using Dan.Common.Models;
 using Dan.Core.Exceptions;
 using Dan.Core.Helpers;
@@ -10,7 +11,7 @@
 
 using ConsentRequirement = Nadobe.Common.Models.ConsentRequirement;
 
-namespace Dan.Core.UnitTest
+namespace Dan.Core.UnitTest.Services
 {
     [TestClass]
     public class RequirementValidationServiceTest
@@ -772,6 +773,140 @@ public async Task ReferenceTest_Failed_ConsentReferenceIsNull()
             Assert.IsTrue(errorList.Count == 1);
             Assert.IsTrue(errorList[0].Contains("The request requires a valid consent reference but none is provided"));
         }
+        
+        [TestMethod]
+        [DataRow("12345678", null)] // Custom subject
+        [DataRow("03065001488", PartyParser.NorwegianIcd)] // Already found subject in pre step
+        [DataRow("03065001488", PartyParser.SchemeIso6523ActorIdUpis)] // Already found subject in pre step
+        [DataRow("03065001488", PartyParser.SchemeNorwegianSsn)] // Already found subject in pre step
+        public async Task CustomSubjectTest_Success(string subject, string scheme)
+        {
+            // Arrange
+            var authRequest = GetAuthRequest(subject, "requestor");
+            authRequest.SubjectParty = new Party
+            {
+                Id = "12345678",
+                Scheme = scheme
+            };
+
+            var req = new Dictionary<string, List<Requirement>>
+            {
+                ["ec1"] =
+                [
+                    new CustomSubjectRequirement()
+                    {
+                        SubjectRegex = @"^\d{1,8}$"
+                    }
+                ]
+            };
+
+            var svc = new RequirementValidationService(_mockAltinnServiceOwnerApiService, _mockEntityRegistryService, _mockRequestContextService);
+            
+            // Act
+            var errorList = await svc.ValidateRequirements(req, authRequest);
+
+            // Assert
+            errorList.Should().BeEmpty();
+        }
+        
+        [TestMethod]
+        public async Task CustomSubjectTest_MismatchFormat_ShouldThrow()
+        {
+            // Arrange
+            var authRequest = GetAuthRequest("123456789", "requestor");
+            authRequest.SubjectParty = new Party
+            {
+                Id = "12345678",
+                Scheme = null
+            };
+
+            var req = new Dictionary<string, List<Requirement>>
+            {
+                ["ec1"] =
+                [
+                    new CustomSubjectRequirement()
+                    {
+                        SubjectRegex = @"^\d{1,8}$",
+                        SubjectRegexDescription = "Description"
+                    }
+                ]
+            };
+
+            var svc = new RequirementValidationService(_mockAltinnServiceOwnerApiService, _mockEntityRegistryService, _mockRequestContextService);
+            
+            // Act
+            var errorList = await svc.ValidateRequirements(req, authRequest);
+
+            // Assert
+            errorList.Should().HaveCount(1);
+            errorList.Should().Contain("ec1: Subject does not match custom subject format: Description");
+        }
+        
+        [TestMethod]
+        public async Task CustomSubjectTest_MissingSubject_ShouldThrow()
+        {
+            // Arrange
+            var authRequest = GetAuthRequest("", "requestor");
+            authRequest.SubjectParty = new Party
+            {
+                Id = "",
+                Scheme = null
+            };
+
+            var req = new Dictionary<string, List<Requirement>>
+            {
+                ["ec1"] =
+                [
+                    new CustomSubjectRequirement()
+                    {
+                        SubjectRegex = @"^\d{1,8}$",
+                        SubjectRegexDescription = "Description"
+                    }
+                ]
+            };
+
+            var svc = new RequirementValidationService(_mockAltinnServiceOwnerApiService, _mockEntityRegistryService, _mockRequestContextService);
+            
+            // Act
+            var errorList = await svc.ValidateRequirements(req, authRequest);
+
+            // Assert
+            errorList.Should().HaveCount(1);
+            errorList.Should().Contain("ec1: Subject is missing");
+        }
+        
+        [TestMethod]
+        public async Task CustomSubjectTest_MissingRegex_ShouldThrow()
+        {
+            // Arrange
+            var authRequest = GetAuthRequest("12345", "requestor");
+            authRequest.SubjectParty = new Party
+            {
+                Id = "12345",
+                Scheme = null
+            };
+
+            var req = new Dictionary<string, List<Requirement>>
+            {
+                ["ec1"] =
+                [
+                    new CustomSubjectRequirement()
+                    {
+                        SubjectRegex = "",
+                        SubjectRegexDescription = "Description"
+                    }
+                ]
+            };
+
+            var svc = new RequirementValidationService(_mockAltinnServiceOwnerApiService, _mockEntityRegistryService, _mockRequestContextService);
+            
+            // Act
+            var errorList = await svc.ValidateRequirements(req, authRequest);
+
+            // Assert
+            errorList.Should().HaveCount(1);
+            errorList.Should().Contain("ec1: Missing custom subject format regex");
+        }
 
         private Requirement GetWhiteListRequirement(List<string> owners, List<string> subjects, List<string> requestors)
         {
diff --git a/Dan.Core/Services/AuthorizationRequestValidatorService.cs b/Dan.Core/Services/AuthorizationRequestValidatorService.cs
index d983307..8c443c9 100644
--- a/Dan.Core/Services/AuthorizationRequestValidatorService.cs
+++ b/Dan.Core/Services/AuthorizationRequestValidatorService.cs
@@ -1,6 +1,5 @@
 using Dan.Common;
 using Dan.Common.Enums;
-using Dan.Common.Interfaces;
 using Dan.Common.Models;
 using Dan.Core.Config;
 using Dan.Core.Exceptions;
@@ -57,9 +56,22 @@ public async Task Validate(AuthorizationRequest? authorizationRequest)
         _authRequest = authorizationRequest ?? throw new InvalidAuthorizationRequestException();
         _registeredEvidenceCodes = await _availableEvidenceCodesService.GetAvailableEvidenceCodes();
         _evidenceCodesFromRequest = _registeredEvidenceCodes.Where(r => _authRequest.EvidenceRequests.Any(x => x.EvidenceCodeName == r.EvidenceCodeName)).ToList();
+        
+        var requirements = _evidenceCodesFromRequest.ToDictionary(es => es.EvidenceCodeName, es => es.AuthorizationRequirements);
+        if (authorizationRequest.FromEvidenceHarvester)
+        {
+            foreach (var requirement in requirements.Values)
+            {
+                requirement.RemoveAll(x => x.RequiredOnEvidenceHarvester == false);
+            }
+        }
 
+        var customSubject = requirements.Values.SelectMany(x => x).Any(req =>
+            req.RequirementType is not null &&
+            req.RequirementType.Equals("CustomSubjectRequirement", StringComparison.InvariantCultureIgnoreCase));
+        
         ValidateAndPopulateRequestor();
-        ValidateAndPopulateSubject();
+        ValidateAndPopulateSubject(customSubject);
         ValidateLegalBasisWellFormed();
         ValidateEvidenceRequestWellFormed();
         ValidateEvidenceCodesAreAvailableForServiceContext();
@@ -78,15 +90,6 @@ public async Task Validate(AuthorizationRequest? authorizationRequest)
 
         ValidateLanguageCodes();
 
-        var requirements = _evidenceCodesFromRequest.ToDictionary(es => es.EvidenceCodeName, es => es.AuthorizationRequirements);
-        if (authorizationRequest.FromEvidenceHarvester)
-        {
-            foreach (var requirement in requirements.Values)
-            {
-                requirement.RemoveAll(x => x.RequiredOnEvidenceHarvester == false);
-            }
-        }
-
         var authorizationErrors = await _requirementValidationService.ValidateRequirements(requirements, _authRequest);
         if (authorizationErrors.Count > 0)
         {
@@ -185,22 +188,33 @@ private void ValidateAndPopulateRequestor()
     /// Uses PartyParser on the supplied subject, and populates SubjectParty with it. Overwrites Requestor with norwegian identifier if applicable, else set to null
     /// </summary>
     /// <exception cref="InvalidSubjectException"></exception>
-    private void ValidateAndPopulateSubject()
+    private void ValidateAndPopulateSubject(bool customSubject)
     {
 
         if (_authRequest.Subject == null)
         {
             return;
         }
-
-        Party? party = PartyParser.GetPartyFromIdentifier(_authRequest.Subject, out string? error);
-        if (party == null)
+        
+        var party = PartyParser.GetPartyFromIdentifier(_authRequest.Subject, out var error);
+        if (party == null && !customSubject)
         {
             throw new InvalidSubjectException($"Invalid subject supplied: {error}");
         }
 
-        _authRequest.Subject = party.NorwegianOrganizationNumber ?? party.NorwegianSocialSecurityNumber;
-        _authRequest.SubjectParty = party;
+        if (party != null)
+        {
+            _authRequest.Subject = party.NorwegianOrganizationNumber ?? party.NorwegianSocialSecurityNumber;
+            _authRequest.SubjectParty = party;
+            return;
+        }
+
+        // Custom Subject Validation will be done later in RequirementValidationService
+        var customParty = new Party
+        {
+            Id = _authRequest.Subject
+        };
+        _authRequest.SubjectParty = customParty;
     }
 
     private void ValidateLegalBasisWellFormed()
diff --git a/Dan.Core/Services/RequirementValidationService.cs b/Dan.Core/Services/RequirementValidationService.cs
index 54344c3..e7db1f9 100644
--- a/Dan.Core/Services/RequirementValidationService.cs
+++ b/Dan.Core/Services/RequirementValidationService.cs
@@ -7,7 +7,6 @@
 using System.Collections.Concurrent;
 using System.Diagnostics.CodeAnalysis;
 using System.Text.RegularExpressions;
-using Dan.Common.Interfaces;
 
 namespace Dan.Core.Services;
 
@@ -107,6 +106,7 @@ private async Task<bool> ValidateSingleRequirement(Requirement req, string evide
             AccreditationPartyRequirement r => ValidateAccreditationPartyRequirement(r, _authRequest.Subject, _authRequest.Requestor, _owner, evidenceCodeName),
             ReferenceRequirement r => ValidateReferenceRequirement(r, _authRequest, evidenceCodeName),
             ProvideOwnTokenRequirement r => ValidateProvideOwnTokenRequirement(r, evidenceCodeName),
+            CustomSubjectRequirement r => ValidateCustomSubjectRequirement(r, _authRequest, evidenceCodeName),
             _ => false
         };
 
@@ -569,6 +569,37 @@ private bool ValidateProvideOwnTokenRequirement(Requirement req, string evidence
         return false;
     }
 
+    private bool ValidateCustomSubjectRequirement(CustomSubjectRequirement req, AuthorizationRequest authRequest, string evidenceCodeName)
+    {
+        
+        if (authRequest.SubjectParty.Scheme is PartyParser.SchemeIso6523ActorIdUpis or 
+                                               PartyParser.SchemeNorwegianSsn or 
+                                               PartyParser.NorwegianIcd)
+        {
+            // Subject already parsed as Organisation number or norwegian ssn
+            return true;
+        }
+        if (string.IsNullOrWhiteSpace(authRequest.Subject))
+        {
+            AddError(req, $"Subject is missing", evidenceCodeName);
+            return false;
+        }
+        
+        if (string.IsNullOrEmpty(req.SubjectRegex))
+        {
+            AddError(req, $"Missing custom subject format regex", evidenceCodeName);
+            return false;
+        }
+        
+        var regexMatch = Regex.Match(authRequest.Subject, req.SubjectRegex, RegexOptions.None, TimeSpan.FromSeconds(1));
+        if (regexMatch.Success)
+        {
+            return true;
+        }
+        AddError(req, $"Subject does not match custom subject format: {req.SubjectRegexDescription}", evidenceCodeName);
+        return false;
+    }
+
     private async Task<PartyTypeConstraint> GetPartyType(string? identifier)
     {
         if (identifier == null) return PartyTypeConstraint.Foreign; 
diff --git a/Dan.PluginTest/Config/PluginConstants.cs b/Dan.PluginTest/Config/PluginConstants.cs
index 5abb96f..aa55968 100644
--- a/Dan.PluginTest/Config/PluginConstants.cs
+++ b/Dan.PluginTest/Config/PluginConstants.cs
@@ -14,4 +14,5 @@ public static class PluginConstants
     public const string PluginForward = "PluginForward";
     public const string PluginSettingsTest = "PluginSettingsTest";
     public const string PluginGenericTest = "PluginGenericTest";
+    public const string PluginCustomSubjectTest = "PluginCustomSubjectTest";
 }
\ No newline at end of file
diff --git a/Dan.PluginTest/Metadata.cs b/Dan.PluginTest/Metadata.cs
index d5b5afa..7acf233 100644
--- a/Dan.PluginTest/Metadata.cs
+++ b/Dan.PluginTest/Metadata.cs
@@ -109,6 +109,29 @@ public List<EvidenceCode> GetEvidenceCodes()
                     }
                 ]
             },
+            new EvidenceCode
+            {
+                EvidenceCodeName = PluginConstants.PluginCustomSubjectTest,
+                EvidenceSource = PluginConstants.Source,
+                BelongsToServiceContexts = _serviceContexts,
+                Values =
+                [
+                    new EvidenceValue
+                    {
+                        EvidenceValueName = "default",
+                        ValueType = EvidenceValueType.JsonSchema,
+                        JsonSchemaDefintion = EvidenceValue.SchemaFromObject<DatasetResponse>()
+                    }
+                ],
+                AuthorizationRequirements = 
+                [
+                    new CustomSubjectRequirement
+                    {
+                        SubjectRegex = @"^\d{1,8}$",
+                        SubjectRegexDescription = "A number 1-8 characters long"
+                    }
+                ]
+            },
         ];
     }
 }
\ No newline at end of file
diff --git a/Dan.PluginTest/Plugin.cs b/Dan.PluginTest/Plugin.cs
index 5f23a8c..787819c 100644
--- a/Dan.PluginTest/Plugin.cs
+++ b/Dan.PluginTest/Plugin.cs
@@ -119,6 +119,31 @@ public async Task<HttpResponseData> PluginSettingsTest(
             () => EvidenceValuesPluginSettings(evidenceHarvesterRequest));
     }
     
+    [Function(PluginConstants.PluginCustomSubjectTest)]
+    public async Task<HttpResponseData> PluginCustomSubjectTest(
+        [HttpTrigger(AuthorizationLevel.Function, "post", Route = null)] HttpRequestData req,
+        FunctionContext context)
+    {
+        EvidenceHarvesterRequest? evidenceHarvesterRequest;
+        try
+        {
+            evidenceHarvesterRequest = await req.ReadFromJsonAsync<EvidenceHarvesterRequest>();
+        }
+        catch (Exception e)
+        {
+            _logger.LogError(e,
+                "Exception while attempting to parse request into EvidenceHarvesterRequest: {exceptionType}: {exceptionMessage}",
+                e.GetType().Name, e.Message);
+            throw new EvidenceSourcePermanentClientException(PluginConstants.ErrorInvalidInput,
+                "Unable to parse request", e);
+        }
+
+        var unit = await ccrClientService.IsPublic("923609016", "local");
+        
+        return await EvidenceSourceResponse.CreateResponse(req,
+            () => GetEvidenceValuesDatasetOne(evidenceHarvesterRequest));
+    }
+    
     [Function(PluginConstants.PluginGenericTest)]
     public async Task<HttpResponseData> PluginGenericTest(
         [HttpTrigger(AuthorizationLevel.Function, "post", Route = null)] HttpRequestData req,