diff --git a/InvenTree/build/models.py b/InvenTree/build/models.py
index e843422f63..dd62a3e6da 100644
--- a/InvenTree/build/models.py
+++ b/InvenTree/build/models.py
@@ -473,8 +473,13 @@ class Build(MPTTModel):
 
         return self.getAllocatedQuantity(part) >= self.getRequiredQuantity(part)
 
-    def getRequiredQuantity(self, part):
-        """ Calculate the quantity of <part> required to make this build.
+    def getRequiredQuantity(self, part, output=None):
+        """
+        Calculate the quantity of <part> required to make this build.
+
+        Args:
+            part: The 'Part' archetype reference
+            output: A particular build output (StockItem) (or None to specify the entire build)
         """
 
         try:
@@ -483,27 +488,51 @@ class Build(MPTTModel):
         except PartModels.BomItem.DoesNotExist:
             q = 0
 
-        return q * self.quantity
+        if output:
+            return q * output.quantity
+        else:
+            return q * self.quantity
 
-    def getAllocatedQuantity(self, part):
-        """ Calculate the total number of <part> currently allocated to this build
+    def getAllocatedQuantity(self, part, output=None):
+        """
+        Calculate the total number of <part> currently allocated to this build.
+
+        Args:
+            part: The 'Part' archetype reference
+            output: A particular build output (StockItem) (or None to specify the entire build)
         """
 
-        allocated = BuildItem.objects.filter(build=self.id, stock_item__part=part.id).aggregate(q=Coalesce(Sum('quantity'), 0))
+        allocations = BuildItem.objects.filter(
+            build=self.id,
+            stock_item__part=part.id
+        )
+
+        # Optionally, filter by the specified build output StockItem
+        if output is not None:
+            allocations = allocations.filter(
+                install_into=output
+            )
+
+        allocated = allocations.aggregate(q=Coalesce(Sum('quantity'), 0))
 
         return allocated['q']
 
-    def getUnallocatedQuantity(self, part):
-        """ Calculate the quantity of <part> which still needs to be allocated to this build.
+    def getUnallocatedQuantity(self, part, output=None):
+        """
+        Calculate the quantity of <part> which still needs to be allocated to this build.
 
         Args:
-            Part - the part to be tested
+            part - the part to be tested
+            output - A particular build output (StockItem) (or None to specify the entire build)
 
         Returns:
             The remaining allocated quantity
         """
 
-        return max(self.getRequiredQuantity(part) - self.getAllocatedQuantity(part), 0)
+        required = self.getRequiredQuantity(part, output=output)
+        allocated = self.getAllocatedQuantity(part, output=output)
+
+        return max(required-allocated, 0)
 
     @property
     def required_parts(self):
diff --git a/InvenTree/build/views.py b/InvenTree/build/views.py
index ff1befcfc3..fdea5534fe 100644
--- a/InvenTree/build/views.py
+++ b/InvenTree/build/views.py
@@ -552,6 +552,8 @@ class BuildItemCreate(AjaxCreateView):
             """
             pass
 
+        self.output = None
+
         # If the output stock item is specified, hide the input field
         output_id = form['install_into'].value()
 
@@ -591,11 +593,11 @@ class BuildItemCreate(AjaxCreateView):
                         pass
 
                     # Exclude StockItem objects which are already allocated to this build and part
-                    stock_filter = stock_filter.exclude(
-                        id__in=[
-                            item.stock_item.id for item in BuildItem.objects.filter(build=build_id, stock_item__part=part_id)
-                        ]
-                    )
+                    to_exclude = BuildItem.objects.filter(build=build_id, stock_item__part=part_id)
+                    if self.output:
+                        to_exclude = to_exclude.filter(install_into=self.output)
+
+                    stock_filter = stock_filter.exclude(id__in=[item.stock_item.id for item in to_exclude.all()])
 
             except Part.DoesNotExist:
                 self.part = None
@@ -654,15 +656,26 @@ class BuildItemCreate(AjaxCreateView):
             except Build.DoesNotExist:
                 pass
 
+        # If the output has been specified
+        if output_id:
+            try:
+                output = StockItem.objects.get(pk=output_id)
+                initials['install_into'] = output
+            except (ValueError, StockItem.DoesNotExist):
+                pass
+
+        # Work out how much stock is required
+        if build and part:
+            required_quantity = build.getUnallocatedQuantity(part, output=output)
+        else:
+            required_quantity = None
+
         quantity = self.request.GET.get('quantity', None)
 
         if quantity is not None:
             quantity = float(quantity)
-
-        if quantity is None:
-            # Work out how many parts remain to be alloacted for the build
-            if part:
-                quantity = build.getUnallocatedQuantity(part)
+        elif required_quantity is not None:
+            quantity = required_quantity
                 
         item_id = self.get_param('item')
 
@@ -686,14 +699,6 @@ class BuildItemCreate(AjaxCreateView):
             else:
                 quantity = min(quantity, item.unallocated_quantity())
 
-        # If the output has been specified
-        if output_id:
-            try:
-                output = StockItem.objects.get(pk=output_id)
-                initials['install_into'] = output
-            except (ValueError, StockItem.DoesNotExist):
-                pass
-
         if quantity is not None:
             initials['quantity'] = quantity
 
diff --git a/InvenTree/part/test_bom_item.py b/InvenTree/part/test_bom_item.py
index 91aa95c17c..a518ca1ddc 100644
--- a/InvenTree/part/test_bom_item.py
+++ b/InvenTree/part/test_bom_item.py
@@ -28,10 +28,12 @@ class BomItemTest(TestCase):
         self.assertEqual(self.bob.bom_count, 4)
 
     def test_in_bom(self):
-        parts = self.bob.required_parts()
+        parts = self.bob.getRequiredParts()
 
         self.assertIn(self.orphan, parts)
 
+    # TODO: Tests for multi-level BOMs
+
     def test_used_in(self):
         self.assertEqual(self.bob.used_in_count, 0)
         self.assertEqual(self.orphan.used_in_count, 1)