diff --git a/bytestring/common/src/ByteStringBuilder.kt b/bytestring/common/src/ByteStringBuilder.kt
index 7b0de5d3..734f159f 100644
--- a/bytestring/common/src/ByteStringBuilder.kt
+++ b/bytestring/common/src/ByteStringBuilder.kt
@@ -17,44 +17,61 @@ import kotlin.math.max
  * The builder avoids additional copies and allocations when `size == capacity` when [toByteString] called,
  * thus it's recommended to specify expected [ByteString] size as `initialCapacity` when creating a builder.
  *
- * When a builder runs out of available capacity, a new byte sequence with extended capacity
- * will be allocated and previously written data will be copied into it.
- *
- * @param initialCapacity the initial size of an underlying byte sequence.
- *
  * @sample kotlinx.io.bytestring.samples.ByteStringSamples.builderSample
  * @sample kotlinx.io.bytestring.samples.ByteStringSamples.builderSampleWithoutAdditionalAllocs
  */
-public class ByteStringBuilder(initialCapacity: Int = 0) {
-    private var buffer = ByteArray(initialCapacity)
-    private var offset: Int = 0
+public class ByteStringBuilder private constructor(
+	private var buffer: ByteArray,
+	private var offset: Int,
+	private val extendable: Boolean
+) {
+	private val safeBuffer get() = if (extendable) buffer else buffer.copyOf()
 
     /**
      * The number of bytes being written to this builder.
      */
     public val size: Int
-        get() = offset
+        get() = if (extendable) offset else buffer.size
 
     /**
-     * The number of bytes this builder can store without an extension of an internal buffer.
+     * The number of bytes this builder can store without extending the internal buffer or throwing.
      */
     public val capacity: Int
         get() = buffer.size
 
-    /**
-     * Returns a new [ByteString] wrapping all bytes written to this builder.
-     *
-     * There will be no additional allocations or copying of data when `size == capacity`.
-     */
-    public fun toByteString(): ByteString {
-        if (size == 0) {
-            return ByteString()
-        }
-        if (buffer.size == size) {
-            return ByteString.wrap(buffer)
-        }
-        return ByteString(buffer, 0, size)
-    }
+	/**
+	 * Creates a new [ByteStringBuilder].
+	 *
+	 * If this builder runs out of available capacity,
+	 * a new byte sequence with extended capacity will be allocated and previously written data will be copied to it.
+	 *
+	 * @param initialCapacity the initial size of the underlying byte sequence.
+	 */
+	public constructor(initialCapacity: Int = 0) : this(ByteArray(initialCapacity), 0, true)
+
+	/**
+	 * Creates a new [ByteStringBuilder] on top of an existing [buffer].
+	 *
+	 * This builder throws when running out of available capacity.
+	 *
+	 * @param initialOffset the offset at which the builder will begin overriding bytes.
+	 */
+	public constructor(buffer: ByteArray, initialOffset: Int = 0) : this(buffer, initialOffset, false)
+
+	/**
+	 * Returns a new [ByteString] wrapping all bytes written to this builder.
+	 *
+	 * There will be no additional allocations or copying of data when `size == capacity`.
+	 */
+	public fun toByteString(): ByteString {
+		if (size == 0) {
+			return ByteString()
+		}
+		if (buffer.size == size) {
+			return ByteString.wrap(safeBuffer)
+		}
+		return ByteString(safeBuffer, 0, size)
+	}
 
     /**
      * Append a single byte to this builder.
@@ -62,8 +79,11 @@ public class ByteStringBuilder(initialCapacity: Int = 0) {
      * @param byte the byte to append.
      */
     public fun append(byte: Byte) {
-        ensureCapacity(size + 1)
-        buffer[offset++] = byte
+		val newOffset = offset + 1
+        ensureCapacity(newOffset)
+
+        buffer[offset] = byte
+	    offset = newOffset
     }
 
     /**
@@ -93,6 +113,8 @@ public class ByteStringBuilder(initialCapacity: Int = 0) {
             return
         }
 
+	    if (!extendable) throw IndexOutOfBoundsException("Needed capacity for appending to this non-extendable ByteString builder is $requiredCapacity, exceeding the available $capacity.")
+
         var desiredSize = if (buffer.isEmpty()) 16 else (buffer.size * 1.5).toInt()
         desiredSize = max(desiredSize, requiredCapacity)
         val newBuffer = ByteArray(desiredSize)
diff --git a/bytestring/common/test/ByteStringBuilderTest.kt b/bytestring/common/test/ByteStringBuilderTest.kt
index 2fbb0540..f378436c 100644
--- a/bytestring/common/test/ByteStringBuilderTest.kt
+++ b/bytestring/common/test/ByteStringBuilderTest.kt
@@ -15,6 +15,41 @@ class ByteStringBuilderTest {
     fun emptyString() {
         assertTrue(ByteStringBuilder().toByteString().isEmpty())
         assertTrue(ByteStringBuilder(1024).toByteString().isEmpty())
+        assertTrue(ByteStringBuilder(byteArrayOf()).toByteString().isEmpty())
+    }
+
+    @Test
+    fun customBuffer() {
+        val byteArray = byteArrayOf(125, 126, 127)
+
+        assertEquals(ByteStringBuilder(byteArray).toByteString(), ByteString(byteArray))
+    }
+
+    @Test
+    fun appendByteToCustomBuffer() {
+        ByteStringBuilder(byteArrayOf(125, 126, 127)).apply {
+            append(1)
+            append(2)
+            append(3)
+            assertEquals(toByteString(), ByteString(1, 2, 3))
+
+            assertFailsWith<IndexOutOfBoundsException> {
+                append(4)
+            }
+        }
+    }
+
+    @Test
+    fun appendByteToCustomBufferWithCustomOffset() {
+        ByteStringBuilder(byteArrayOf(125, 126, 127), initialOffset = 1).apply {
+            append(2)
+            append(3)
+            assertEquals(toByteString(), ByteString(125, 2, 3))
+
+            assertFailsWith<IndexOutOfBoundsException> {
+                append(4)
+            }
+        }
     }
 
     @Test