diff --git a/src/economy.c b/src/economy.c
index acccca8f1..c6e0d360a 100644
--- a/src/economy.c
+++ b/src/economy.c
@@ -1101,7 +1101,7 @@ static void allocate_resource(unit * u, const resource_type * rtype, int want)
         for (; mod->flags != 0; ++mod) {
             if (mod->flags & RMF_REQUIREDBUILDING) {
                 struct building *b = inside_building(u);
-                const struct building_type *btype = b ? b->type : NULL;
+                const struct building_type *btype = building_is_active(b) ? b->type : NULL;
                 if (mod->btype && mod->btype != btype) {
                     cmistake(u, u->thisorder, 104, MSG_PRODUCE);
                     return;
@@ -1157,7 +1157,7 @@ static void allocate_resource(unit * u, const resource_type * rtype, int want)
     }
     else {
         struct building *b = inside_building(u);
-        const struct building_type *btype = b ? b->type : NULL;
+        const struct building_type *btype = building_is_active(b) ? b->type : NULL;
 
         if (rdata->modifiers) {
             resource_mod *mod = rdata->modifiers;
@@ -1213,7 +1213,7 @@ static void allocate_resource(unit * u, const resource_type * rtype, int want)
 
     if (rdata->modifiers) {
         struct building *b = inside_building(u);
-        const struct building_type *btype = b ? b->type : NULL;
+        const struct building_type *btype = building_is_active(b) ? b->type : NULL;
 
         resource_mod *mod = rdata->modifiers;
         for (; mod->flags != 0; ++mod) {
@@ -2422,7 +2422,7 @@ static void breedhorses(unit * u)
 {
     int n, c, breed = 0;
     struct building *b = inside_building(u);
-    const struct building_type *btype = b ? b->type : NULL;
+    const struct building_type *btype = building_is_active(b) ? b->type : NULL;
     const struct resource_type *rhorse = get_resourcetype(R_HORSE);
     int horses, effsk;
 
diff --git a/src/kernel/build.c b/src/kernel/build.c
index 924dccf49..e38d76641 100644
--- a/src/kernel/build.c
+++ b/src/kernel/build.c
@@ -448,8 +448,9 @@ int build(unit * u, const construction * ctype, int completed, int want)
             return EBUILDINGREQ;
         }
         b = inside_building(u);
-        if (b == NULL)
+        if (!b || !building_is_active(b)) {
             return EBUILDINGREQ;
+        }
     }
 
     if (type->skill != NOSKILL) {
@@ -462,7 +463,7 @@ int build(unit * u, const construction * ctype, int completed, int want)
             return ENEEDSKILL;
 
         effsk = basesk;
-        if (inside_building(u)) {
+        if (building_is_active(u->building) && inside_building(u)) {
             effsk = skillmod(u->building->type->attribs, u, u->region, type->skill,
                 effsk, SMF_PRODUCTION);
         }
@@ -558,7 +559,7 @@ int build(unit * u, const construction * ctype, int completed, int want)
                 int need, prebuilt;
                 int canuse = get_pooled(u, rtype, GET_DEFAULT, INT_MAX);
 
-                if (inside_building(u)) {
+                if (building_is_active(u->building) && inside_building(u)) {
                     canuse = matmod(u->building->type->attribs, u, rtype, canuse);
                 }
 
@@ -597,8 +598,9 @@ int build(unit * u, const construction * ctype, int completed, int want)
                     required(completed + n, type->reqsize, type->materials[c].number);
                 int multi = 1;
                 int canuse = 100;       /* normalization */
-                if (inside_building(u))
+                if (building_is_active(u->building) && inside_building(u)) {
                     canuse = matmod(u->building->type->attribs, u, rtype, canuse);
+                }
                 if (canuse < 0)
                     return canuse;        /* pass errors to caller */
                 canuse = matmod(type->attribs, u, rtype, canuse);
diff --git a/src/kernel/building.c b/src/kernel/building.c
index da4f58867..d63ada88e 100644
--- a/src/kernel/building.c
+++ b/src/kernel/building.c
@@ -642,6 +642,10 @@ region *building_getregion(const building * b)
     return b->region;
 }
 
+bool building_is_active(const struct building *b) {
+    return b && fval(b, BLD_WORKING);
+}
+
 void building_setregion(building * b, region * r)
 {
     building **blist = &b->region->buildings;
diff --git a/src/kernel/building.h b/src/kernel/building.h
index b4bb23efa..c0796b352 100644
--- a/src/kernel/building.h
+++ b/src/kernel/building.h
@@ -78,6 +78,7 @@ extern "C" {
     } building_type;
 
     extern struct quicklist *buildingtypes;
+    extern struct attrib_type at_building_action;
 
     building_type *bt_get_or_create(const char *name);
     const building_type *bt_find(const char *name);
@@ -163,7 +164,7 @@ extern "C" {
     extern void building_set_owner(struct unit * u);
     extern void building_update_owner(struct building * bld);
 
-    extern struct attrib_type at_building_action;
+    bool building_is_active(const struct building *b);
 
 #ifdef WDW_PYRAMID
     extern int wdw_pyramid_level(const struct building *b);
diff --git a/src/kernel/unit.c b/src/kernel/unit.c
index 5e9b39705..23919eeac 100644
--- a/src/kernel/unit.c
+++ b/src/kernel/unit.c
@@ -1072,11 +1072,7 @@ void transfermen(unit * u, unit * dst, int n)
 
 struct building *inside_building(const struct unit *u)
 {
-    if (u->building == NULL)
-        return NULL;
-
-    if (!fval(u->building, BLD_WORKING)) {
-        /* Unterhalt nicht bezahlt */
+    if (!u->building || !building_is_active(u->building)) {
         return NULL;
     }
     else if (u->building->size < u->building->type->maxsize) {
diff --git a/src/kernel/unit.h b/src/kernel/unit.h
index 606bccbcf..89fb90d9b 100644
--- a/src/kernel/unit.h
+++ b/src/kernel/unit.h
@@ -183,36 +183,36 @@ extern "C" {
     void make_zombie(unit * u);
 
     /* see resolve.h */
-    extern int resolve_unit(variant data, void *address);
-    extern void write_unit_reference(const struct unit *u, struct storage *store);
-    extern variant read_unit_reference(struct storage *store);
+    int resolve_unit(variant data, void *address);
+    void write_unit_reference(const struct unit *u, struct storage *store);
+    variant read_unit_reference(struct storage *store);
 
-    extern bool leave(struct unit *u, bool force);
-    extern bool can_leave(struct unit *u);
+    bool leave(struct unit *u, bool force);
+    bool can_leave(struct unit *u);
 
-    extern void u_set_building(struct unit * u, struct building * b);
-    extern void u_set_ship(struct unit * u, struct ship * sh);
-    extern void leave_ship(struct unit * u);
-    extern void leave_building(struct unit * u);
+    void u_set_building(struct unit * u, struct building * b);
+    void u_set_ship(struct unit * u, struct ship * sh);
+    void leave_ship(struct unit * u);
+    void leave_building(struct unit * u);
 
-    extern void set_leftship(struct unit *u, struct ship *sh);
-    extern struct ship *leftship(const struct unit *);
-    extern bool can_survive(const struct unit *u, const struct region *r);
-    extern void move_unit(struct unit *u, struct region *target,
-    struct unit **ulist);
+    void set_leftship(struct unit *u, struct ship *sh);
+    struct ship *leftship(const struct unit *);
+    bool can_survive(const struct unit *u, const struct region *r);
+    void move_unit(struct unit *u, struct region *target,
+        struct unit **ulist);
 
-    extern struct building *inside_building(const struct unit *u);
+    struct building *inside_building(const struct unit *u);
 
     /* cleanup code for this module */
-    extern void free_units(void);
-    extern struct faction *dfindhash(int no);
-    extern void u_setfaction(struct unit *u, struct faction *f);
-    extern void set_number(struct unit *u, int count);
+    void free_units(void);
+    struct faction *dfindhash(int no);
+    void u_setfaction(struct unit *u, struct faction *f);
+    void set_number(struct unit *u, int count);
 
-    extern bool learn_skill(struct unit *u, skill_t sk, double chance);
+    bool learn_skill(struct unit *u, skill_t sk, double chance);
 
-    extern int invisible(const struct unit *target, const struct unit *viewer);
-    extern void free_unit(struct unit *u);
+    int invisible(const struct unit *target, const struct unit *viewer);
+    void free_unit(struct unit *u);
 
     extern void name_unit(struct unit *u);
     extern struct unit *create_unit(struct region *r1, struct faction *f,
diff --git a/src/laws.c b/src/laws.c
index d3cdcaba4..3b4cc6fca 100755
--- a/src/laws.c
+++ b/src/laws.c
@@ -3545,7 +3545,7 @@ void monthly_healing(void)
                 double maxheal = _max(u->number, umhp / 20.0);
                 int addhp;
                 struct building *b = inside_building(u);
-                const struct building_type *btype = b ? b->type : NULL;
+                const struct building_type *btype = building_is_active(b) ? b->type : NULL;
                 if (btype == bt_find("inn")) {
                     p *= 1.5;
                 }
diff --git a/src/magic.c b/src/magic.c
index 52d85f435..e0a5ca921 100644
--- a/src/magic.c
+++ b/src/magic.c
@@ -1025,7 +1025,7 @@ spellpower(region * r, unit * u, const spell * sp, int cast_level, struct order
     else {
         /* Bonus durch Magieturm und gesegneten Steinkreis */
         struct building *b = inside_building(u);
-        const struct building_type *btype = b ? b->type : NULL;
+        const struct building_type *btype = building_is_active(b) ? b->type : NULL;
         if (btype && btype->flags & BTF_MAGIC) ++force;
     }
 
@@ -1170,7 +1170,7 @@ double magic_resistance(unit * target)
     /* Bonus durch Gebäude */
     {
         struct building *b = inside_building(target);
-        const struct building_type *btype = b ? b->type : NULL;
+        const struct building_type *btype = building_is_active(b) ? b->type : NULL;
 
         /* gesegneter Steinkreis gibt 30% dazu */
         if (btype)
@@ -1284,7 +1284,7 @@ bool fumble(region * r, unit * u, const spell * sp, int cast_grade)
     int fumble_chance, rnd = 0;
     int effsk = effskill(u, SK_MAGIC, r);
     struct building *b = inside_building(u);
-    const struct building_type *btype = b ? b->type : NULL;
+    const struct building_type *btype = building_is_active(b) ? b->type : NULL;
     int fumble_enabled = get_param_int(global.parameters, "magic.fumble.enable", 1);
     sc_mage * mage;
 
@@ -1468,7 +1468,7 @@ void regenerate_aura(void)
                 auramax = max_spellpoints(r, u);
                 if (aura < auramax) {
                     struct building *b = inside_building(u);
-                    const struct building_type *btype = b ? b->type : NULL;
+                    const struct building_type *btype = building_is_active(b) ? b->type : NULL;
                     reg_aura = regeneration(u);
 
                     /* Magierturm erhöht die Regeneration um 75% */
diff --git a/src/study.c b/src/study.c
index 5edba9168..b798277ca 100644
--- a/src/study.c
+++ b/src/study.c
@@ -175,6 +175,13 @@ static int study_days(unit * student, skill_t sk)
     return student->number * speed;
 }
 
+static building *active_building(const unit *u, const struct building_type *btype) {
+    if (u->building && u->building->type == btype && building_is_active(u->building)) {
+        return inside_building(u);
+    }
+    return 0;
+}
+
 static int
 teach_unit(unit * teacher, unit * student, int nteaching, skill_t sk,
 bool report, int *academy)
@@ -205,8 +212,7 @@ bool report, int *academy)
     n = _min(n, nteaching);
 
     if (n != 0) {
-        struct building *b = inside_building(teacher);
-        const struct building_type *btype = b ? b->type : NULL;
+        const struct building_type *btype = bt_find("academy");
         int index = 0;
 
         if (teach == NULL) {
@@ -229,8 +235,7 @@ bool report, int *academy)
 
         /* Solange Akademien groessenbeschraenkt sind, sollte Lehrer und
          * Student auch in unterschiedlichen Gebaeuden stehen duerfen */
-        if (btype == bt_find("academy")
-            && student->building && student->building->type == bt_find("academy")) {
+        if (active_building(teacher, btype) && active_building(student, btype)) {
             int j = study_cost(student, sk);
             j = _max(50, j * 2);
             /* kann Einheit das zahlen? */
@@ -542,7 +547,7 @@ int study_cmd(unit * u, order * ord)
     int speed_rule = (study_rule_t)get_param_int(global.parameters, "study.speedup", 0);
     static int learn_newskills = -1;
     struct building *b = inside_building(u);
-    const struct building_type *btype = b ? b->type : NULL;
+    const struct building_type *btype = building_is_active(b) ? b->type : NULL;
 
     if (learn_newskills < 0) {
         const char *str = get_param(global.parameters, "study.newskills");
@@ -606,7 +611,7 @@ int study_cmd(unit * u, order * ord)
     }
     /* Akademie: */
     b = inside_building(u);
-    btype = b ? b->type : NULL;
+    btype = building_is_active(b) ? b->type : NULL;
 
     if (btype && btype == bt_find("academy")) {
         studycost = _max(50, studycost * 2);