diff --git a/Sources/AblyChat/Errors.swift b/Sources/AblyChat/Errors.swift
new file mode 100644
index 0000000..0586ae8
--- /dev/null
+++ b/Sources/AblyChat/Errors.swift
@@ -0,0 +1,67 @@
+import Ably
+
+/**
+ The error domain used for the ``Ably.ARTErrorInfo`` error instances thrown by the Ably Chat SDK.
+
+ See ``ErrorCode`` for the possible ``ARTErrorInfo.code`` values.
+ */
+public let errorDomain: NSString = "AblyChatErrorDomain"
+
+/**
+ The error codes for errors in the ``errorDomain`` error domain.
+ */
+public enum ErrorCode: Int {
+    /// ``Rooms.get(roomID:options:)`` was called with a different set of room options than was used on a previous call. You must first release the existing room instance using ``Rooms.release(roomID:)``.
+    ///
+    /// TODO this code is a guess, revisit in https://github.com/ably-labs/ably-chat-swift/issues/32
+    case inconsistentRoomOptions = 1
+
+    /// The ``ARTErrorInfo.statusCode`` that should be returned for this error.
+    internal var statusCode: Int {
+        // TODO: These are currently a guess, revisit in https://github.com/ably-labs/ably-chat-swift/issues/32
+        switch self {
+        case .inconsistentRoomOptions:
+            400
+        }
+    }
+}
+
+/**
+ The errors thrown by the Chat SDK.
+
+ This type exists in addition to ``ErrorCode`` to allow us to attach metadata which can be incorporated into the error’s `localizedDescription`.
+ */
+internal enum ChatError {
+    case inconsistentRoomOptions(requested: RoomOptions, existing: RoomOptions)
+
+    /// The ``ARTErrorInfo.code`` that should be returned for this error.
+    internal var code: ErrorCode {
+        switch self {
+        case .inconsistentRoomOptions:
+            .inconsistentRoomOptions
+        }
+    }
+
+    /// The ``ARTErrorInfo.localizedDescription`` that should be returned for this error.
+    internal var localizedDescription: String {
+        switch self {
+        case let .inconsistentRoomOptions(requested, existing):
+            "Rooms.get(roomID:options:) was called with a different set of room options than was used on a previous call. You must first release the existing room instance using Rooms.release(roomID:). Requested options: \(requested), existing options: \(existing)"
+        }
+    }
+}
+
+internal extension ARTErrorInfo {
+    convenience init(chatError: ChatError) {
+        var userInfo: [String: Any] = [:]
+        // TODO: copied and pasted from implementation of -[ARTErrorInfo createWithCode:status:message:requestId:] because there’s no way to pass domain; revisit in https://github.com/ably-labs/ably-chat-swift/issues/32. Also the ARTErrorInfoStatusCode variable in ably-cocoa is not public.
+        userInfo["ARTErrorInfoStatusCode"] = chatError.code.statusCode
+        userInfo[NSLocalizedDescriptionKey] = chatError.localizedDescription
+
+        self.init(
+            domain: errorDomain as String,
+            code: chatError.code.rawValue,
+            userInfo: userInfo
+        )
+    }
+}
diff --git a/Sources/AblyChat/Room.swift b/Sources/AblyChat/Room.swift
index 286821a..01d1350 100644
--- a/Sources/AblyChat/Room.swift
+++ b/Sources/AblyChat/Room.swift
@@ -1,3 +1,5 @@
+@preconcurrency import Ably
+
 public protocol Room: AnyObject, Sendable {
     var roomID: String { get }
     var messages: any Messages { get }
@@ -14,3 +16,49 @@ public protocol Room: AnyObject, Sendable {
     func detach() async throws
     var options: RoomOptions { get }
 }
+
+public final class DefaultRoom: Room {
+    public let roomID: String
+    public let options: RoomOptions
+
+    // Exposed for testing.
+    internal let realtime: any ARTRealtimeProtocol
+
+    internal init(realtime: any ARTRealtimeProtocol, roomID: String, options: RoomOptions) {
+        self.realtime = realtime
+        self.roomID = roomID
+        self.options = options
+    }
+
+    public var messages: any Messages {
+        fatalError("Not yet implemented")
+    }
+
+    public var presence: any Presence {
+        fatalError("Not yet implemented")
+    }
+
+    public var reactions: any RoomReactions {
+        fatalError("Not yet implemented")
+    }
+
+    public var typing: any Typing {
+        fatalError("Not yet implemented")
+    }
+
+    public var occupancy: any Occupancy {
+        fatalError("Not yet implemented")
+    }
+
+    public var status: any RoomStatus {
+        fatalError("Not yet implemented")
+    }
+
+    public func attach() async throws {
+        fatalError("Not yet implemented")
+    }
+
+    public func detach() async throws {
+        fatalError("Not yet implemented")
+    }
+}
diff --git a/Sources/AblyChat/RoomOptions.swift b/Sources/AblyChat/RoomOptions.swift
index a927bfc..ca7b597 100644
--- a/Sources/AblyChat/RoomOptions.swift
+++ b/Sources/AblyChat/RoomOptions.swift
@@ -1,6 +1,6 @@
 import Foundation
 
-public struct RoomOptions: Sendable {
+public struct RoomOptions: Sendable, Equatable {
     public var presence: PresenceOptions?
     public var typing: TypingOptions?
     public var reactions: RoomReactionsOptions?
@@ -14,7 +14,7 @@ public struct RoomOptions: Sendable {
     }
 }
 
-public struct PresenceOptions: Sendable {
+public struct PresenceOptions: Sendable, Equatable {
     public var enter = true
     public var subscribe = true
 
@@ -24,7 +24,7 @@ public struct PresenceOptions: Sendable {
     }
 }
 
-public struct TypingOptions: Sendable {
+public struct TypingOptions: Sendable, Equatable {
     public var timeout: TimeInterval = 10
 
     public init(timeout: TimeInterval = 10) {
@@ -32,10 +32,10 @@ public struct TypingOptions: Sendable {
     }
 }
 
-public struct RoomReactionsOptions: Sendable {
+public struct RoomReactionsOptions: Sendable, Equatable {
     public init() {}
 }
 
-public struct OccupancyOptions: Sendable {
+public struct OccupancyOptions: Sendable, Equatable {
     public init() {}
 }
diff --git a/Sources/AblyChat/Rooms.swift b/Sources/AblyChat/Rooms.swift
index f09478a..aa39b2e 100644
--- a/Sources/AblyChat/Rooms.swift
+++ b/Sources/AblyChat/Rooms.swift
@@ -1,5 +1,64 @@
+@preconcurrency import Ably
+
 public protocol Rooms: AnyObject, Sendable {
     func get(roomID: String, options: RoomOptions) throws -> any Room
     func release(roomID: String) async throws
     var clientOptions: ClientOptions { get }
 }
+
+internal final class DefaultRooms: Rooms {
+    private let realtime: ARTRealtimeProtocol
+    private let storage = Storage()
+
+    internal init(realtime: ARTRealtimeProtocol) {
+        self.realtime = realtime
+    }
+
+    /// Thread-safe storage of the set of rooms.
+    private class Storage: @unchecked Sendable {
+        /// Mutex used to synchronize access to ``rooms``.
+        private let lock = NSLock()
+
+        /// The set of rooms, keyed by room ID. Only access whilst holding ``lock``.
+        private var rooms: [String: DefaultRoom] = [:]
+
+        /// If there is an existing room with the given ID, returns it. Else creates a new room with the given ID and options.
+        public func getOrCreate(realtime: any ARTRealtimeProtocol, roomID: String, options: RoomOptions) -> DefaultRoom {
+            let room: DefaultRoom
+            lock.lock()
+            // CHA-RC1b
+            if let existingRoom = rooms[roomID] {
+                room = existingRoom
+            } else {
+                room = DefaultRoom(realtime: realtime, roomID: roomID, options: options)
+                rooms[roomID] = room
+            }
+            lock.unlock()
+            return room
+        }
+    }
+
+    internal func get(roomID: String, options: RoomOptions) throws -> any Room {
+        let room = storage.getOrCreate(
+            realtime: realtime,
+            roomID: roomID,
+            options: options
+        )
+
+        if room.options != options {
+            throw ARTErrorInfo(
+                chatError: .inconsistentRoomOptions(requested: options, existing: room.options)
+            )
+        }
+
+        return room
+    }
+
+    internal func release(roomID _: String) async throws {
+        fatalError("Not yet implemented")
+    }
+
+    internal var clientOptions: ClientOptions {
+        fatalError("Not yet implemented")
+    }
+}
diff --git a/Tests/AblyChatTests/AblyChatTests.swift b/Tests/AblyChatTests/AblyChatTests.swift
index c8c5c0b..fbda0fe 100644
--- a/Tests/AblyChatTests/AblyChatTests.swift
+++ b/Tests/AblyChatTests/AblyChatTests.swift
@@ -3,6 +3,6 @@ import XCTest
 
 final class AblyChatTests: XCTestCase {
     func testExample() throws {
-        XCTAssertNoThrow(DefaultChatClient(realtime: MockRealtime(key: ""), clientOptions: ClientOptions()))
+        XCTAssertNoThrow(DefaultChatClient(realtime: MockRealtime.create(), clientOptions: ClientOptions()))
     }
 }
diff --git a/Tests/AblyChatTests/DefaultRoomsTests.swift b/Tests/AblyChatTests/DefaultRoomsTests.swift
new file mode 100644
index 0000000..796750e
--- /dev/null
+++ b/Tests/AblyChatTests/DefaultRoomsTests.swift
@@ -0,0 +1,64 @@
+@testable import AblyChat
+import XCTest
+
+class DefaultRoomsTests: XCTestCase {
+    // @spec CHA-RC1a
+    func test_get_returnsRoomWithGivenID() throws {
+        // Given: an instance of DefaultRooms
+        let realtime = MockRealtime.create()
+        let rooms = DefaultRooms(realtime: realtime)
+
+        // When: get(roomID:options:) is called
+        let roomID = "basketball"
+        let options = RoomOptions()
+        let room = try rooms.get(roomID: roomID, options: options)
+
+        // Then: It returns a DefaultRoom instance that uses the same Realtime instance, with the given ID and options
+        let defaultRoom = try XCTUnwrap(room as? DefaultRoom)
+        XCTAssertIdentical(defaultRoom.realtime, realtime)
+        XCTAssertEqual(defaultRoom.roomID, roomID)
+        XCTAssertEqual(defaultRoom.options, options)
+    }
+
+    // @spec CHA-RC1b
+    func test_get_returnsExistingRoomWithGivenID() throws {
+        // Given: an instance of DefaultRooms, on which get(roomID:options:) has already been called with a given ID
+        let realtime = MockRealtime.create()
+        let rooms = DefaultRooms(realtime: realtime)
+
+        let roomID = "basketball"
+        let options = RoomOptions()
+        let firstRoom = try rooms.get(roomID: roomID, options: options)
+
+        // When: get(roomID:options:) is called with the same room ID
+        let secondRoom = try rooms.get(roomID: roomID, options: options)
+
+        // Then: It returns the same room object
+        XCTAssertIdentical(secondRoom, firstRoom)
+    }
+
+    // @spec CHA-RC1c
+    func test_get_throwsErrorWhenOptionsDoNotMatch() throws {
+        // Given: an instance of DefaultRooms, on which get(roomID:options:) has already been called with a given ID and options
+        let realtime = MockRealtime.create()
+        let rooms = DefaultRooms(realtime: realtime)
+
+        let roomID = "basketball"
+        let options = RoomOptions()
+        _ = try rooms.get(roomID: roomID, options: options)
+
+        // When: get(roomID:options:) is called with the same ID but different options
+        let differentOptions = RoomOptions(presence: .init(subscribe: false))
+
+        let caughtError: Error?
+        do {
+            _ = try rooms.get(roomID: roomID, options: differentOptions)
+            caughtError = nil
+        } catch {
+            caughtError = error
+        }
+
+        // Then: It throws an inconsistentRoomOptions error
+        try assertIsChatError(caughtError, withCode: .inconsistentRoomOptions)
+    }
+}
diff --git a/Tests/AblyChatTests/Helpers/Helpers.swift b/Tests/AblyChatTests/Helpers/Helpers.swift
new file mode 100644
index 0000000..669c23e
--- /dev/null
+++ b/Tests/AblyChatTests/Helpers/Helpers.swift
@@ -0,0 +1,15 @@
+import Ably
+@testable import AblyChat
+import XCTest
+
+/**
+ Asserts that a given optional `Error` is an `ARTErrorInfo` in the chat error domain with a given code.
+ */
+func assertIsChatError(_ maybeError: (any Error)?, withCode code: AblyChat.ErrorCode, file: StaticString = #filePath, line: UInt = #line) throws {
+    let error = try XCTUnwrap(maybeError, "Expected an error", file: file, line: line)
+    let ablyError = try XCTUnwrap(error as? ARTErrorInfo, "Expected an ARTErrorInfo", file: file, line: line)
+
+    XCTAssertEqual(ablyError.domain, AblyChat.errorDomain as String, file: file, line: line)
+    XCTAssertEqual(ablyError.code, code.rawValue, file: file, line: line)
+    XCTAssertEqual(ablyError.statusCode, code.statusCode, file: file, line: line)
+}
diff --git a/Tests/AblyChatTests/Mocks/MockRealtime.swift b/Tests/AblyChatTests/Mocks/MockRealtime.swift
index f49b06c..efdd449 100644
--- a/Tests/AblyChatTests/Mocks/MockRealtime.swift
+++ b/Tests/AblyChatTests/Mocks/MockRealtime.swift
@@ -15,6 +15,15 @@ class MockRealtime: NSObject, ARTRealtimeProtocol {
 
     required init(token _: String) {}
 
+    /**
+     Creates an instance of MockRealtime.
+
+     This exists to give a convenient way to create an instance, because `init` is marked as unavailable in `ARTRealtimeProtocol`.
+     */
+    static func create() -> MockRealtime {
+        MockRealtime(key: "")
+    }
+
     func time(_: @escaping ARTDateTimeCallback) {
         fatalError("Not implemented")
     }