diff --git a/src/town.h b/src/town.h
--- a/src/town.h
+++ b/src/town.h
@@ -35,8 +35,8 @@ static const uint INVALID_TOWN = 0xFFFF;
 
 static const uint TOWN_GROWTH_WINTER = 0xFFFFFFFE; ///< The town only needs this cargo in the winter (any amount)
 static const uint TOWN_GROWTH_DESERT = 0xFFFFFFFF; ///< The town needs the cargo for growth when on desert (any amount)
-static const uint16 TOWN_GROW_RATE_CUSTOM      = 0x8000; ///< If this mask is applied to Town::growth_rate, the grow_counter will not be calculated by the system (but assumed to be set by scripts)
-static const uint16 TOWN_GROW_RATE_CUSTOM_NONE = 0xFFFF; ///< Special value for Town::growth_rate to disable town growth.
+static const uint16 TOWN_GROWTH_RATE_NONE = 0xFFFF; ///< Special value for Town::growth_rate to disable town growth.
+static const uint16 MAX_TOWN_GROWTH_TICKS = 930; ///< Max amount of original town ticks that still fit into uint16, about equal to UINT16_MAX / TOWN_GROWTH_TICKS but sligtly less to simplify calculations
 
 typedef Pool<Town, TownID, 64, 64000> TownPool;
 extern TownPool _town_pool;
@@ -165,6 +165,7 @@ enum TownFlags {
 	TOWN_IS_GROWING     = 0,   ///< Conditions for town growth are met. Grow according to Town::growth_rate.
 	TOWN_HAS_CHURCH     = 1,   ///< There can be only one church by town.
 	TOWN_HAS_STADIUM    = 2,   ///< There can be only one stadium by town.
+	TOWN_CUSTOM_GROWTH  = 3,   ///< Growth rate is controlled by GS.
 };
 
 CommandCost CheckforTownRating(DoCommandFlag flags, Town *t, TownRatingCheckType type);
@@ -194,7 +195,6 @@ uint GetMaskOfTownActions(int *nump, Com
 bool GenerateTowns(TownLayout layout);
 const CargoSpec *FindFirstCargoWithTownEffect(TownEffect effect);
 
-
 /** Town actions of a company. */
 enum TownActions {
 	TACT_NONE             = 0x00, ///< Empty action set.
@@ -284,6 +284,15 @@ void MakeDefaultName(T *obj)
 	obj->town_cn = (uint16)next; // set index...
 }
 
+/*
+ * Converts original town ticks counters to plain game ticks. Note that
+ * tick 0 is a valid tick so actual amount is one more than the counter value.
+ */
+static inline uint16 TownTicksToGameTicks(uint16 ticks) {
+	return (min(ticks, MAX_TOWN_GROWTH_TICKS) + 1) * TOWN_GROWTH_TICKS - 1;
+}
+
+
 extern uint32 _town_cargoes_accepted;
 
 #endif /* TOWN_H */