diff --git a/macros.h b/macros.h
--- a/macros.h
+++ b/macros.h
@@ -146,8 +146,10 @@ static inline void swap_tile(TileIndex *
 
 /// Fetch n bits starting at bit s from x
 #define GB(x, s, n) (((x) >> (s)) & ((1 << (n)) - 1))
-/// Set n bits in x starting at bit s to d
+/// Set n bits starting at bit s in x to d
 #define SB(x, s, n, d) ((x) = ((x) & ~(((1 << (n)) - 1) << (s))) | ((d) << (s)))
+/// Add i to the n bits starting at bit s in x
+#define AB(x, s, n, i) ((x) = ((x) & ~(((1 << (n)) - 1) << (s))) | (((x) + ((i) << (s))) & (((1 << (n)) - 1) << (s))))
 
 /**
  * ROtate x Left/Right by n (must be >= 0)
diff --git a/town_cmd.c b/town_cmd.c
--- a/town_cmd.c
+++ b/town_cmd.c
@@ -180,7 +180,7 @@ static void AnimateTile_Town(TileIndex t
 			i = (Random()&7) - 1;
 		} while (i < 0 || i == 1 || i*6==old);
 
-		_m[tile].m5 = (_m[tile].m5 & ~0x3F) | i;
+		SB(_m[tile].m5, 0, 6, i);
 	}
 
 	a = _m[tile].owner&0x7F;
@@ -255,18 +255,12 @@ uint32 GetWorldPopulation(void)
 
 static void MakeSingleHouseBigger(TileIndex tile)
 {
-	byte b;
-
 	assert(IsTileType(tile, MP_HOUSE));
 
-	b = _m[tile].m5;
-	if (b & 0x80)
-		return;
+	if (_m[tile].m5 & 0x80) return;
 
-	_m[tile].m5 = (b & 0xC0) | ((b+1)&7);
-
-	if ((_m[tile].m5&7) != 0)
-		return;
+	AB(_m[tile].m5, 0, 3, 1);
+	if (GB(_m[tile].m5, 0, 3) != 0) return;
 
 	_m[tile].m3 = _m[tile].m3 + 0x40;
 
diff --git a/tree_cmd.c b/tree_cmd.c
--- a/tree_cmd.c
+++ b/tree_cmd.c
@@ -485,11 +485,8 @@ static void TileLoop_Trees(TileIndex til
 	TileLoopClearHelper(tile);
 
 	/* increase counter */
-	{
-		uint16 m2 = _m[tile].m2;
-		_m[tile].m2 = m2 = (m2 & 0xF0) | ((m2 + 1) & 0xF);
-		if ((m2 & 0xF) != 0) return;
-	}
+	AB(_m[tile].m2, 0, 4, 1);
+	if (GB(_m[tile].m2, 0, 4) != 0) return;
 
 	m5 = _m[tile].m5;
 	if (GB(m5, 0, 3) == 3) {