diff --git a/.changeset/quiet-moles-live.md b/.changeset/quiet-moles-live.md
new file mode 100644
index 00000000..a07cd5bf
--- /dev/null
+++ b/.changeset/quiet-moles-live.md
@@ -0,0 +1,10 @@
+---
+"@siteimprove/alfa-test-utils": patch
+"@siteimprove/alfa-frontier": patch
+"@siteimprove/alfa-command": patch
+"@siteimprove/alfa-crawler": patch
+"@siteimprove/alfa-scraper": patch
+"@siteimprove/alfa-assert": patch
+---
+
+**hanged:** Classes that do not implement the Singleton pattern now have `protected` constructor and can be extended.
diff --git a/packages/alfa-assert/src/asserter.ts b/packages/alfa-assert/src/asserter.ts
index 588748d6..fc5d985b 100644
--- a/packages/alfa-assert/src/asserter.ts
+++ b/packages/alfa-assert/src/asserter.ts
@@ -20,7 +20,7 @@ export class Asserter<I, T extends Hashable, Q extends Question.Metadata, S> {
   private readonly _handlers: Array<Handler<I, T, Q, S>>;
   private readonly _options: Asserter.Options<I, T, Q, S>;
 
-  private constructor(
+  protected constructor(
     rules: Array<Rule<I, T, Q, S>>,
     handlers: Array<Handler<I, T, Q, S>>,
     options: Asserter.Options<I, T, Q, S>
diff --git a/packages/alfa-assert/src/assertion.ts b/packages/alfa-assert/src/assertion.ts
index b062b5f9..0aecfd90 100644
--- a/packages/alfa-assert/src/assertion.ts
+++ b/packages/alfa-assert/src/assertion.ts
@@ -35,7 +35,7 @@ export class Assertion<I, T extends Hashable, Q extends Question.Metadata, S> {
   private readonly _handlers: Array<Handler<I, T, Q, S>>;
   private readonly _options: Assertion.Options<I, T, Q, S>;
 
-  private constructor(
+  protected constructor(
     input: I,
     rules: Array<Rule<I, T, Q, S>>,
     handlers: Array<Handler<I, T, Q, S>>,
diff --git a/packages/alfa-command/src/argument.ts b/packages/alfa-command/src/argument.ts
index 43cb7d98..c77b749c 100644
--- a/packages/alfa-command/src/argument.ts
+++ b/packages/alfa-command/src/argument.ts
@@ -40,7 +40,7 @@ export class Argument<T = unknown>
   private readonly _options: Argument.Options;
   private readonly _parse: Argument.Parser<T>;
 
-  private constructor(
+  protected constructor(
     name: string,
     description: string,
     options: Argument.Options,
diff --git a/packages/alfa-command/src/command.ts b/packages/alfa-command/src/command.ts
index 0a319f77..81fb5561 100644
--- a/packages/alfa-command/src/command.ts
+++ b/packages/alfa-command/src/command.ts
@@ -78,7 +78,7 @@ export class Command<
   private readonly _parent: Option<Command>;
   private readonly _run: Command.Runner<F, A>;
 
-  private constructor(
+  protected constructor(
     name: string,
     version: string,
     description: string,
diff --git a/packages/alfa-command/src/flag.ts b/packages/alfa-command/src/flag.ts
index f59ae89a..8b040a1d 100644
--- a/packages/alfa-command/src/flag.ts
+++ b/packages/alfa-command/src/flag.ts
@@ -42,7 +42,7 @@ export class Flag<T = unknown> implements Functor<T>, Serializable<Flag.JSON> {
   private readonly _options: Flag.Options;
   private readonly _parse: Flag.Parser<T, [Predicate<string>]>;
 
-  private constructor(
+  protected constructor(
     name: string,
     description: string,
     options: Flag.Options,
@@ -393,7 +393,7 @@ export namespace Flag {
     private readonly _value: T;
     private readonly _parse: Flag.Parser<T>;
 
-    private constructor(value: T, parse: Flag.Parser<T>) {
+    protected constructor(value: T, parse: Flag.Parser<T>) {
       this._value = value;
       this._parse = parse;
     }
diff --git a/packages/alfa-crawler/src/crawler.ts b/packages/alfa-crawler/src/crawler.ts
index 291620ff..7596d088 100644
--- a/packages/alfa-crawler/src/crawler.ts
+++ b/packages/alfa-crawler/src/crawler.ts
@@ -34,7 +34,7 @@ export class Crawler {
 
   private readonly _scraper: Scraper;
 
-  private constructor(scraper: Scraper) {
+  protected constructor(scraper: Scraper) {
     this._scraper = scraper;
   }
 
diff --git a/packages/alfa-frontier/src/frontier.ts b/packages/alfa-frontier/src/frontier.ts
index d7530b0c..2d2fc993 100644
--- a/packages/alfa-frontier/src/frontier.ts
+++ b/packages/alfa-frontier/src/frontier.ts
@@ -29,7 +29,7 @@ export class Frontier implements Equatable, Serializable {
   private readonly _scope: URL;
   private readonly _items: Array<Item>;
 
-  private constructor(scope: URL, items: Array<Item>) {
+  protected constructor(scope: URL, items: Array<Item>) {
     this._scope = scope;
     this._items = items;
   }
@@ -243,7 +243,7 @@ class Item implements Equatable, Serializable {
   private readonly _aliases: Array<URL>;
   private _state: State;
 
-  private constructor(url: URL, aliases: Array<URL>, state: State) {
+  protected constructor(url: URL, aliases: Array<URL>, state: State) {
     this._url = url;
     this._aliases = aliases;
     this._state = state;
diff --git a/packages/alfa-scraper/src/archive.ts b/packages/alfa-scraper/src/archive.ts
index 5fdc5a1e..5bc68ef6 100644
--- a/packages/alfa-scraper/src/archive.ts
+++ b/packages/alfa-scraper/src/archive.ts
@@ -17,7 +17,7 @@ export class Archive implements Equatable, Serializable {
   private readonly _path: string;
   private readonly _format: Archive.Format;
 
-  private constructor(path: string, format: Archive.Format) {
+  protected constructor(path: string, format: Archive.Format) {
     this._path = path;
     this._format = format;
   }
diff --git a/packages/alfa-scraper/src/credentials.ts b/packages/alfa-scraper/src/credentials.ts
index 85ac62c6..b094c351 100644
--- a/packages/alfa-scraper/src/credentials.ts
+++ b/packages/alfa-scraper/src/credentials.ts
@@ -14,7 +14,7 @@ export class Credentials implements Equatable, Serializable {
   private readonly _username: string;
   private readonly _password: string;
 
-  private constructor(username: string, password: string) {
+  protected constructor(username: string, password: string) {
     this._username = username;
     this._password = password;
   }
diff --git a/packages/alfa-scraper/src/scraper.ts b/packages/alfa-scraper/src/scraper.ts
index 3d500430..a9326d34 100644
--- a/packages/alfa-scraper/src/scraper.ts
+++ b/packages/alfa-scraper/src/scraper.ts
@@ -63,7 +63,7 @@ export class Scraper {
 
   private readonly _browser: puppeteer.Browser;
 
-  private constructor(browser: puppeteer.Browser) {
+  protected constructor(browser: puppeteer.Browser) {
     this._browser = browser;
   }
 
diff --git a/packages/alfa-scraper/src/screenshot.ts b/packages/alfa-scraper/src/screenshot.ts
index 2bb250d9..6d03409a 100644
--- a/packages/alfa-scraper/src/screenshot.ts
+++ b/packages/alfa-scraper/src/screenshot.ts
@@ -17,7 +17,7 @@ export class Screenshot implements Equatable, Serializable {
   private readonly _path: string;
   private readonly _type: Screenshot.Type;
 
-  private constructor(path: string, type: Screenshot.Type) {
+  protected constructor(path: string, type: Screenshot.Type) {
     this._path = path;
     this._type = type;
   }
@@ -68,7 +68,7 @@ export namespace Screenshot {
 
       private readonly _background: boolean;
 
-      private constructor(background: boolean) {
+      protected constructor(background: boolean) {
         this._background = background;
       }
 
@@ -107,7 +107,7 @@ export namespace Screenshot {
 
       private readonly _quality: number;
 
-      private constructor(quality: number) {
+      protected constructor(quality: number) {
         this._quality = quality;
       }
 
diff --git a/packages/alfa-test-utils/src/audit/audit.ts b/packages/alfa-test-utils/src/audit/audit.ts
index 0bd74f9b..36d0b821 100644
--- a/packages/alfa-test-utils/src/audit/audit.ts
+++ b/packages/alfa-test-utils/src/audit/audit.ts
@@ -37,7 +37,7 @@ export class Audit implements json.Serializable<Audit.JSON> {
   private readonly _resultAggregates: Audit.ResultAggregates;
   private readonly _durations: Performance.Durations;
 
-  private constructor(
+  protected constructor(
     page: Page,
     outcomes: Map<string, Sequence<alfaOutcome>>,
     resultAggregates: Audit.ResultAggregates,
diff --git a/packages/alfa-test-utils/src/report/logging.ts b/packages/alfa-test-utils/src/report/logging.ts
index fffa76a1..af644588 100644
--- a/packages/alfa-test-utils/src/report/logging.ts
+++ b/packages/alfa-test-utils/src/report/logging.ts
@@ -26,7 +26,7 @@ export class Logging implements Equatable, json.Serializable<Logging.JSON> {
   private readonly _title: string;
   private readonly _logs: Sequence<Logging>;
 
-  private constructor(title: string, logs: Sequence<Logging>) {
+  protected constructor(title: string, logs: Sequence<Logging>) {
     this._title = title;
     this._logs = logs;
   }