diff --git a/csharp/ql/lib/semmle/code/csharp/Callable.qll b/csharp/ql/lib/semmle/code/csharp/Callable.qll
index 49a2271b27c..1bdfb008144 100644
--- a/csharp/ql/lib/semmle/code/csharp/Callable.qll
+++ b/csharp/ql/lib/semmle/code/csharp/Callable.qll
@@ -221,6 +221,23 @@ class Callable extends Parameterizable, ExprOrStmtParent, @callable {
 
   /** Gets a `Call` that has this callable as a target. */
   Call getACall() { this = result.getTarget() }
+
+  /** Holds if this callable is declared in an extension type. */
+  predicate isInExtension() { this.getDeclaringType() instanceof ExtensionType }
+}
+
+/**
+ * A callable that is declared as an extension.
+ *
+ * Either an extension method (`ExtensionMethod`), an extension operator
+ * (`ExtensionOperator`) or an extension accessor (`ExtensionAccessor`).
+ */
+abstract class ExtensionCallable extends Callable {
+  /** Gets the type being extended by this method. */
+  pragma[noinline]
+  Type getExtendedType() { result = this.getDeclaringType().(ExtensionType).getExtendedType() }
+
+  override string getAPrimaryQlClass() { result = "ExtensionCallable" }
 }
 
 /**
@@ -267,8 +284,11 @@ class Method extends Callable, Virtualizable, Attributable, @method {
 
   override Location getALocation() { method_location(this.getUnboundDeclaration(), result) }
 
+  /** Holds if this method is a classic extension method. */
+  predicate isClassicExtensionMethod() { this.getParameter(0).hasExtensionMethodModifier() }
+
   /** Holds if this method is an extension method. */
-  predicate isExtensionMethod() { this.getParameter(0).hasExtensionMethodModifier() }
+  predicate isExtensionMethod() { this.isClassicExtensionMethod() or this.isInExtension() }
 
   /** Gets the type of the `params` parameter of this method, if any. */
   Type getParamsType() {
@@ -296,7 +316,17 @@ class Method extends Callable, Virtualizable, Attributable, @method {
 }
 
 /**
- * An extension method, for example
+ * An extension method.
+ *
+ * Either a classic extension method (`ClassicExtensionMethod`) or an extension
+ * type extension method (`ExtensionTypeExtensionMethod`).
+ */
+abstract class ExtensionMethod extends ExtensionCallable, Method {
+  override string getAPrimaryQlClass() { result = "ExtensionMethod" }
+}
+
+/**
+ *  An extension method, for example
  *
  * ```csharp
  * static bool IsDefined(this Widget w) {
@@ -304,16 +334,28 @@ class Method extends Callable, Virtualizable, Attributable, @method {
  * }
  * ```
  */
-class ExtensionMethod extends Method {
-  ExtensionMethod() { this.isExtensionMethod() }
+class ClassicExtensionMethod extends ExtensionMethod {
+  ClassicExtensionMethod() { this.isClassicExtensionMethod() }
+
+  pragma[noinline]
+  override Type getExtendedType() { result = this.getParameter(0).getType() }
 
   override predicate isStatic() { any() }
+}
 
-  /** Gets the type being extended by this method. */
-  pragma[noinline]
-  Type getExtendedType() { result = this.getParameter(0).getType() }
-
-  override string getAPrimaryQlClass() { result = "ExtensionMethod" }
+/**
+ * An extension method declared in an extension type, for example `IsNullOrEmpty` in
+ *
+ * ```csharp
+ * static class MyExtensions {
+ *   extension(string s) {
+ *     public bool IsNullOrEmpty() { ... }
+ *   }
+ * }
+ * ```
+ */
+class ExtensionTypeExtensionMethod extends ExtensionMethod {
+  ExtensionTypeExtensionMethod() { this.isInExtension() }
 }
 
 /**
@@ -536,6 +578,21 @@ class RecordCloneMethod extends Method {
   }
 }
 
+/**
+ * An extension operator, for example `*` in
+ *
+ * ```csharp
+ * static class MyExtensions {
+ *   extension(string s) {
+ *     public static string operator *(int s1, string s2) { ... }
+ *   }
+ * }
+ * ```
+ */
+class ExtensionOperator extends ExtensionCallable, Operator {
+  ExtensionOperator() { this.isInExtension() }
+}
+
 /**
  * A user-defined unary operator - an operator taking one operand.
  *
diff --git a/csharp/ql/lib/semmle/code/csharp/Property.qll b/csharp/ql/lib/semmle/code/csharp/Property.qll
index e651639b631..d3e65def671 100644
--- a/csharp/ql/lib/semmle/code/csharp/Property.qll
+++ b/csharp/ql/lib/semmle/code/csharp/Property.qll
@@ -260,6 +260,21 @@ class Property extends DeclarationWithGetSetAccessors, @property {
   override string getAPrimaryQlClass() { result = "Property" }
 }
 
+/**
+ * An extension property, for example `FirstChar` in
+ *
+ * ```csharp
+ * static class MyExtensions {
+ *   extension(string s) {
+ *     public char FirstChar { get { ... } }
+ *   }
+ * }
+ * ```
+ */
+class ExtensionProperty extends Property {
+  ExtensionProperty() { this.getDeclaringType() instanceof ExtensionType }
+}
+
 /**
  * An indexer, for example `string this[int i]` on line 2 in
  *
@@ -413,6 +428,22 @@ class Accessor extends Callable, Modifiable, Attributable, Overridable, @callabl
   override string toString() { result = this.getName() }
 }
 
+/**
+ *  An extension accessor. Either a getter (`Getter`) or a setter (`Setter`) of an
+ *  extension property, for example `get` in
+ *
+ * ```csharp
+ * static class MyExtensions {
+ *   extension(string s) {
+ *     public char FirstChar { get { ... } }
+ *   }
+ * }
+ * ```
+ */
+class ExtensionAccessor extends ExtensionCallable, Accessor {
+  ExtensionAccessor() { this.getDeclaringType() instanceof ExtensionType }
+}
+
 /**
  * A `get` accessor, for example `get { return p; }` in
  *