diff --git a/generic/arch.cc b/generic/arch.cc
index 6f2bd5df..4fc07613 100644
--- a/generic/arch.cc
+++ b/generic/arch.cc
@@ -78,7 +78,24 @@ void Arch::addBel(IdString name, IdString type, Loc loc, bool gb)
     bel_ids_by_type[type].push_back(name);
 
     bel_by_loc[loc] = name;
+
+    if (bels_by_tile.size() <= loc.x)
+        bels_by_tile.resize(loc.x + 1);
+
+    if (bels_by_tile[loc.x].size() <= loc.y)
+        bels_by_tile[loc.x].resize(loc.y + 1);
+
     bels_by_tile[loc.x][loc.y].push_back(name);
+
+    if (tileDimZ.size() <= loc.x)
+        tileDimZ.resize(loc.x + 1);
+
+    if (tileDimZ[loc.x].size() <= loc.y)
+        tileDimZ[loc.x].resize(loc.y + 1);
+
+    gridDimX = std::max(gridDimX, loc.x + 1);
+    gridDimY = std::max(gridDimY, loc.x + 1);
+    tileDimZ[loc.x][loc.y] = std::max(tileDimZ[loc.x][loc.y], loc.z + 1);
 }
 
 void Arch::addBelInput(IdString bel, IdString name, IdString wire)
diff --git a/generic/arch.h b/generic/arch.h
index a77dca68..5f0df988 100644
--- a/generic/arch.h
+++ b/generic/arch.h
@@ -87,11 +87,14 @@ struct Arch : BaseCtx
     std::unordered_map<IdString, std::vector<IdString>> bel_ids_by_type;
 
     std::unordered_map<Loc, BelId> bel_by_loc;
-    std::unordered_map<int, std::unordered_map<int, std::vector<BelId>>> bels_by_tile;
+    std::vector<std::vector<std::vector<BelId>>> bels_by_tile;
 
     std::unordered_map<DecalId, std::vector<GraphicElement>> decal_graphics;
     DecalXY frame_decalxy;
 
+    int gridDimX, gridDimY;
+    std::vector<std::vector<int>> tileDimZ;
+
     float grid_distance_to_delay;
 
     void addWire(IdString name, int x, int y);
@@ -131,6 +134,10 @@ struct Arch : BaseCtx
     BelType belTypeFromId(IdString id) const { return id; }
     PortPin portPinFromId(IdString id) const { return id; }
 
+    int getGridDimX() const { return gridDimX; }
+    int getGridDimY() const { return gridDimY; }
+    int getTileDimZ(int x, int y) const { return tileDimZ[x][y]; }
+
     BelId getBelByName(IdString name) const;
     IdString getBelName(BelId bel) const;
     Loc getBelLocation(BelId bel) const;
diff --git a/ice40/arch.h b/ice40/arch.h
index 3ae6ce65..cd699a0c 100644
--- a/ice40/arch.h
+++ b/ice40/arch.h
@@ -377,6 +377,12 @@ struct Arch : BaseCtx
 
     // -------------------------------------------------
 
+    int getGridDimX() const { return 34; }
+    int getGridDimY() const { return 34; }
+    int getTileDimZ(int, int) const { return 8; }
+
+    // -------------------------------------------------
+
     BelId getBelByName(IdString name) const;
 
     IdString getBelName(BelId bel) const