diff --git a/common/place_common.cc b/common/place_common.cc
index 9522624e..1d4427ac 100644
--- a/common/place_common.cc
+++ b/common/place_common.cc
@@ -306,10 +306,18 @@ class ConstraintLegaliseWorker
                 currentLoc = ctx->getBelLocation(cell->bel);
             if (cell->constr_x == cell->UNCONSTR)
                 xRootSearch = IncreasingDiameterSearch(currentLoc.x, 0, ctx->getGridDimX() - 1);
+            else
+                xRootSearch = IncreasingDiameterSearch(cell->constr_x);
+
             if (cell->constr_y == cell->UNCONSTR)
                 yRootSearch = IncreasingDiameterSearch(currentLoc.y, 0, ctx->getGridDimY() - 1);
+            else
+                yRootSearch = IncreasingDiameterSearch(cell->constr_y);
+
             if (cell->constr_z == cell->UNCONSTR)
                 zRootSearch = IncreasingDiameterSearch(currentLoc.z, 0, ctx->getTileDimZ(currentLoc.x, currentLoc.y));
+            else
+                zRootSearch = IncreasingDiameterSearch(cell->constr_z);
             while (!xRootSearch.done()) {
                 Loc rootLoc;
                 rootLoc.x = xRootSearch.get();
@@ -352,6 +360,7 @@ class ConstraintLegaliseWorker
                         }
                         ctx->bindBel(target, cp.first, STRENGTH_LOCKED);
                     }
+                    NPNR_ASSERT(constraints_satisfied(cell));
                     return true;
                 }
             }
diff --git a/ice40/chains.cc b/ice40/chains.cc
index b1ad1007..54e284af 100644
--- a/ice40/chains.cc
+++ b/ice40/chains.cc
@@ -260,7 +260,8 @@ class ChainConstrainer
                 log_info("Placing carry chain starting at '%s'\n", chain.cells.front()->name.c_str(ctx));
 
             // Place carry chain
-            chain.cells.at(0)->constr_abs_z = 0;
+            chain.cells.at(0)->constr_abs_z = true;
+            chain.cells.at(0)->constr_z = 0;
             for (int i = 1; i < int(chain.cells.size()); i++) {
                 chain.cells.at(i)->constr_x = 0;
                 chain.cells.at(i)->constr_y = (i / 8);