diff --git a/internal/world/decoration_region.go b/internal/world/decoration_region.go
new file mode 100644
index 0000000..c6050a8
--- /dev/null
+++ b/internal/world/decoration_region.go
@@ -0,0 +1,148 @@
+package world
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"regionio/internal/worldgen"
+)
+
+// decorationRegion is the mutable terrain view used while replaying source
+// chunk feature passes. A target needs base terrain through radius two: its nine
+// possible source chunks each inspect biomes in their own radius-one region.
+type decorationRegion struct {
+	chunks  map[[2]int32]*Chunk
+	sourceX int32
+	sourceZ int32
+}
+
+func newDecorationRegion(chunks []*Chunk) (*decorationRegion, error) {
+	region := &decorationRegion{chunks: make(map[[2]int32]*Chunk, len(chunks))}
+	for _, chunk := range chunks {
+		if chunk == nil {
+			return nil, fmt.Errorf("world: nil chunk in decoration region")
+		}
+		key := [2]int32{chunk.X, chunk.Z}
+		if _, exists := region.chunks[key]; exists {
+			return nil, fmt.Errorf("world: duplicate decoration chunk (%d,%d)", chunk.X, chunk.Z)
+		}
+		region.chunks[key] = chunk
+	}
+	return region, nil
+}
+
+func (r *decorationRegion) setSource(cx, cz int32) error {
+	if _, ok := r.chunks[[2]int32{cx, cz}]; !ok {
+		return fmt.Errorf("world: decoration source (%d,%d) unavailable", cx, cz)
+	}
+	r.sourceX, r.sourceZ = cx, cz
+	return nil
+}
+
+func (r *decorationRegion) chunkAtBlock(x, z int) *Chunk {
+	return r.chunks[[2]int32{int32(x >> 4), int32(z >> 4)}]
+}
+
+func (r *decorationRegion) getBlock(x, y, z int) uint16 {
+	if y < MinY || y >= MinY+WorldHeight {
+		return StateAir
+	}
+	chunk := r.chunkAtBlock(x, z)
+	if chunk == nil {
+		return StateAir
+	}
+	return chunk.GetBlock(x&15, y, z&15)
+}
+
+func (r *decorationRegion) setBlock(x, y, z int, state uint16) bool {
+	if y < MinY || y >= MinY+WorldHeight {
+		return false
+	}
+	cx, cz := int32(x>>4), int32(z>>4)
+	if abs32(cx-r.sourceX) > 1 || abs32(cz-r.sourceZ) > 1 {
+		return false
+	}
+	chunk := r.chunks[[2]int32{cx, cz}]
+	if chunk == nil {
+		return false
+	}
+	chunk.SetBlock(x&15, y, z&15, state)
+	return true
+}
+
+// heightAt mirrors WorldGenRegion.getHeight: one above the highest matching
+// block, or MinY when the column has no match.
+func (r *decorationRegion) heightAt(kind string, x, z int) int {
+	for y := MinY + WorldHeight - 1; y >= MinY; y-- {
+		state := r.getBlock(x, y, z)
+		match := false
+		switch kind {
+		case "WORLD_SURFACE", "WORLD_SURFACE_WG":
+			match = state != StateAir
+		case "OCEAN_FLOOR", "OCEAN_FLOOR_WG":
+			match = stateFlags(state)&flagBlocksMotion != 0
+		case "MOTION_BLOCKING":
+			match = blocksMotionOrFluid(state)
+		case "MOTION_BLOCKING_NO_LEAVES":
+			match = blocksMotionNoLeaves(state)
+		}
+		if match {
+			return y + 1
+		}
+	}
+	return MinY
+}
+
+func (r *decorationRegion) getBiome(x, y, z int) (uint16, bool) {
+	chunk := r.chunkAtBlock(x, z)
+	if chunk == nil {
+		return 0, false
+	}
+	return chunk.GetBiome(x&15, y, z&15), true
+}
+
+func (r *decorationRegion) sourceBiomes() []string {
+	seen := make(map[uint16]bool)
+	var names []string
+	for cx := r.sourceX - 1; cx <= r.sourceX+1; cx++ {
+		for cz := r.sourceZ - 1; cz <= r.sourceZ+1; cz++ {
+			chunk := r.chunks[[2]int32{cx, cz}]
+			if chunk == nil {
+				continue
+			}
+			for si := 0; si < SectionCount; si++ {
+				for bx := 0; bx < biomeCellsXZ; bx++ {
+					for by := 0; by < biomeCellsXZ; by++ {
+						for bz := 0; bz < biomeCellsXZ; bz++ {
+							id := chunk.GetBiome(bx*biomeCellSize, MinY+si*16+by*biomeCellSize, bz*biomeCellSize)
+							if !seen[id] {
+								seen[id] = true
+								names = append(names, biomeNameByID(id))
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+	return names
+}
+
+func (r *decorationRegion) placementContext(biomeAllows func(worldgen.FeaturePosition) bool) worldgen.PlacementContext {
+	return worldgen.PlacementContext{
+		MinY:        MinY,
+		Height:      WorldHeight,
+		BiomeAllows: biomeAllows,
+		HeightAt:    r.heightAt,
+		BlockPredicate: func(predicate json.RawMessage, position worldgen.FeaturePosition) (bool, error) {
+			return false, fmt.Errorf("world: unsupported block predicate at (%d,%d,%d): %s", position.X, position.Y, position.Z, predicate)
+		},
+	}
+}
+
+func abs32(value int32) int32 {
+	if value < 0 {
+		return -value
+	}
+	return value
+}
diff --git a/internal/world/decoration_region_test.go b/internal/world/decoration_region_test.go
new file mode 100644
index 0000000..6cff182
--- /dev/null
+++ b/internal/world/decoration_region_test.go
@@ -0,0 +1,84 @@
+package world
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestDecorationRegionUsesAbsoluteNegativeCoordinates(t *testing.T) {
+	chunk := NewChunk(-1, -1, BiomePlains)
+	chunk.SetBlock(15, 12, 15, StateStone)
+	region, err := newDecorationRegion([]*Chunk{chunk})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := region.getBlock(-1, 12, -1); got != StateStone {
+		t.Fatalf("block (-1,12,-1) = %d, want stone", got)
+	}
+	if biome, ok := region.getBiome(-1, 12, -1); !ok || biome != BiomePlains {
+		t.Fatalf("biome = %d, %v", biome, ok)
+	}
+}
+
+func TestDecorationRegionEnforcesSourceWriteRadius(t *testing.T) {
+	var chunks []*Chunk
+	for cx := int32(-2); cx <= 2; cx++ {
+		chunks = append(chunks, NewChunk(cx, 0, BiomePlains))
+	}
+	region, err := newDecorationRegion(chunks)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := region.setSource(0, 0); err != nil {
+		t.Fatal(err)
+	}
+	if !region.setBlock(-1, 20, 0, StateStone) || !region.setBlock(31, 20, 0, StateDirt) {
+		t.Fatal("write inside source radius rejected")
+	}
+	if region.setBlock(-17, 20, 0, StateStone) || region.setBlock(32, 20, 0, StateStone) {
+		t.Fatal("write outside source radius accepted")
+	}
+	if got := region.getBlock(31, 20, 0); got != StateDirt {
+		t.Fatalf("shared region mutation = %d, want dirt", got)
+	}
+}
+
+func TestDecorationRegionHeightmapsMatchPlacementSemantics(t *testing.T) {
+	chunk := NewChunk(0, 0, BiomePlains)
+	chunk.SetBlock(2, 30, 3, StateStone)
+	chunk.SetBlock(2, 31, 3, StateWater)
+	region, err := newDecorationRegion([]*Chunk{chunk})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := region.heightAt("WORLD_SURFACE", 2, 3); got != 32 {
+		t.Fatalf("WORLD_SURFACE = %d, want 32", got)
+	}
+	if got := region.heightAt("OCEAN_FLOOR", 2, 3); got != 31 {
+		t.Fatalf("OCEAN_FLOOR = %d, want 31", got)
+	}
+	if got := region.heightAt("MOTION_BLOCKING", 2, 3); got != 32 {
+		t.Fatalf("MOTION_BLOCKING = %d, want 32", got)
+	}
+	if got := region.heightAt("WORLD_SURFACE", 4, 5); got != MinY {
+		t.Fatalf("empty height = %d, want %d", got, MinY)
+	}
+}
+
+func TestDecorationRegionSourceBiomesUseThreeByThreeChunks(t *testing.T) {
+	center := NewChunk(0, 0, BiomePlains)
+	east := NewChunk(1, 0, biomeIDByName("minecraft:desert"))
+	outside := NewChunk(2, 0, biomeIDByName("minecraft:forest"))
+	region, err := newDecorationRegion([]*Chunk{center, east, outside})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := region.setSource(0, 0); err != nil {
+		t.Fatal(err)
+	}
+	got := region.sourceBiomes()
+	want := []string{"minecraft:plains", "minecraft:desert"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("source biomes = %v, want %v", got, want)
+	}
+}