diff --git a/MaterialLib/PorousMedium/Permeability/Permeability.h b/MaterialLib/PorousMedium/Permeability/Permeability.h
index 21883a95cfd7fba47cdb6defe8cdeaaf7e02de0f..c34a1780aa977938f4a2332456047e89d35966f9 100644
--- a/MaterialLib/PorousMedium/Permeability/Permeability.h
+++ b/MaterialLib/PorousMedium/Permeability/Permeability.h
@@ -50,10 +50,10 @@ public:
      *  @param variable    A variable with any double type value.
      *  @param temperature Temperature with any double type value.
      */
-    Eigen::MatrixXd const& getValue(const double t,
-                                    ParameterLib::SpatialPosition const& pos,
-                                    const double variable,
-                                    const double temperature) const
+    Eigen::MatrixXd getValue(const double t,
+                             ParameterLib::SpatialPosition const& pos,
+                             const double variable,
+                             const double temperature) const
     {
         (void)variable;
         (void)temperature;
diff --git a/MaterialLib/TwoPhaseModels/TwoPhaseFlowWithPPMaterialProperties.cpp b/MaterialLib/TwoPhaseModels/TwoPhaseFlowWithPPMaterialProperties.cpp
index 5a50c0fd37d0332b516aade4f42e208e44fceb69..070e6218e21ce3faddc32dd3b6c8bce44fd9d72c 100644
--- a/MaterialLib/TwoPhaseModels/TwoPhaseFlowWithPPMaterialProperties.cpp
+++ b/MaterialLib/TwoPhaseModels/TwoPhaseFlowWithPPMaterialProperties.cpp
@@ -118,7 +118,7 @@ double TwoPhaseFlowWithPPMaterialProperties::getGasViscosity(
     return _gas_viscosity->getValue(vars);
 }
 
-Eigen::MatrixXd const& TwoPhaseFlowWithPPMaterialProperties::getPermeability(
+Eigen::MatrixXd TwoPhaseFlowWithPPMaterialProperties::getPermeability(
     const int material_id, const double t,
     const ParameterLib::SpatialPosition& pos, const int /*dim*/) const
 {
diff --git a/MaterialLib/TwoPhaseModels/TwoPhaseFlowWithPPMaterialProperties.h b/MaterialLib/TwoPhaseModels/TwoPhaseFlowWithPPMaterialProperties.h
index b6f2811a39916bdd5a97194b70a5ee377173d9c6..7df516049da41b87584a8231400da12cb46b4c25 100644
--- a/MaterialLib/TwoPhaseModels/TwoPhaseFlowWithPPMaterialProperties.h
+++ b/MaterialLib/TwoPhaseModels/TwoPhaseFlowWithPPMaterialProperties.h
@@ -79,11 +79,10 @@ public:
 
     int getMaterialID(const std::size_t element_id) const;
 
-    Eigen::MatrixXd const& getPermeability(
-        const int material_id,
-        const double t,
-        const ParameterLib::SpatialPosition& pos,
-        const int dim) const;
+    Eigen::MatrixXd getPermeability(const int material_id,
+                                    const double t,
+                                    const ParameterLib::SpatialPosition& pos,
+                                    const int dim) const;
 
     double getPorosity(const int material_id, const double t,
                        const ParameterLib::SpatialPosition& pos, const double p,
diff --git a/ProcessLib/LiquidFlow/LiquidFlowMaterialProperties.cpp b/ProcessLib/LiquidFlow/LiquidFlowMaterialProperties.cpp
index 908b2dd0f2b6898ede45a19d575823b5505e8821..bc5680c8e61e37cb3cec91ee5c4c904ca1a03eef 100644
--- a/ProcessLib/LiquidFlow/LiquidFlowMaterialProperties.cpp
+++ b/ProcessLib/LiquidFlow/LiquidFlowMaterialProperties.cpp
@@ -95,7 +95,7 @@ double LiquidFlowMaterialProperties::getMassCoefficient(
     return porosity * drho_dp / rho + storage;
 }
 
-Eigen::MatrixXd const& LiquidFlowMaterialProperties::getPermeability(
+Eigen::MatrixXd LiquidFlowMaterialProperties::getPermeability(
     const int material_id, const double t,
     const ParameterLib::SpatialPosition& pos, const int /*dim*/) const
 {
diff --git a/ProcessLib/LiquidFlow/LiquidFlowMaterialProperties.h b/ProcessLib/LiquidFlow/LiquidFlowMaterialProperties.h
index 09eb9d85e2472764ff684d19f2ad574d3fcaaf51..6e909a15ccf30158c46e57b1685085f92987c169 100644
--- a/ProcessLib/LiquidFlow/LiquidFlowMaterialProperties.h
+++ b/ProcessLib/LiquidFlow/LiquidFlowMaterialProperties.h
@@ -91,11 +91,10 @@ public:
                               const double porosity_variable,
                               const double storage_variable) const;
 
-    Eigen::MatrixXd const& getPermeability(
-        const int material_id,
-        const double t,
-        const ParameterLib::SpatialPosition& pos,
-        const int dim) const;
+    Eigen::MatrixXd getPermeability(const int material_id,
+                                    const double t,
+                                    const ParameterLib::SpatialPosition& pos,
+                                    const int dim) const;
 
     double getLiquidDensity(const double p, const double T) const;
 
diff --git a/ProcessLib/RichardsFlow/RichardsFlowMaterialProperties.cpp b/ProcessLib/RichardsFlow/RichardsFlowMaterialProperties.cpp
index 62200a5ebac41865653538284cd165bbb78ebff1..a4917f16e851a6730c0bf939d313ff302a43cdc6 100644
--- a/ProcessLib/RichardsFlow/RichardsFlowMaterialProperties.cpp
+++ b/ProcessLib/RichardsFlow/RichardsFlowMaterialProperties.cpp
@@ -85,7 +85,7 @@ double RichardsFlowMaterialProperties::getFluidViscosity(const double p,
         MaterialLib::Fluid::FluidPropertyType::Viscosity, vars);
 }
 
-Eigen::MatrixXd const& RichardsFlowMaterialProperties::getPermeability(
+Eigen::MatrixXd RichardsFlowMaterialProperties::getPermeability(
     const int material_id, const double t,
     const ParameterLib::SpatialPosition& pos, const int /*dim*/) const
 {
diff --git a/ProcessLib/RichardsFlow/RichardsFlowMaterialProperties.h b/ProcessLib/RichardsFlow/RichardsFlowMaterialProperties.h
index 7b7f8683921510ca2c2b08a33a58b939d543455e..085423fda36050a2a068f3a0fa44d39bc7d33230 100644
--- a/ProcessLib/RichardsFlow/RichardsFlowMaterialProperties.h
+++ b/ProcessLib/RichardsFlow/RichardsFlowMaterialProperties.h
@@ -66,11 +66,10 @@ public:
 
     int getMaterialID(const std::size_t element_id);
 
-    Eigen::MatrixXd const& getPermeability(
-        const int material_id,
-        const double t,
-        const ParameterLib::SpatialPosition& pos,
-        const int dim) const;
+    Eigen::MatrixXd getPermeability(const int material_id,
+                                    const double t,
+                                    const ParameterLib::SpatialPosition& pos,
+                                    const int dim) const;
 
     double getPorosity(const int material_id, const double t,
                        const ParameterLib::SpatialPosition& pos, const double p,
diff --git a/ProcessLib/TwoPhaseFlowWithPP/TwoPhaseFlowWithPPMaterialProperties.cpp b/ProcessLib/TwoPhaseFlowWithPP/TwoPhaseFlowWithPPMaterialProperties.cpp
index eb5047f9c34a060e52748b047f2175f7f5db07e3..2d3ec26b6665d8cacca3ffe7b74777b14cf6d70d 100644
--- a/ProcessLib/TwoPhaseFlowWithPP/TwoPhaseFlowWithPPMaterialProperties.cpp
+++ b/ProcessLib/TwoPhaseFlowWithPP/TwoPhaseFlowWithPPMaterialProperties.cpp
@@ -120,7 +120,7 @@ double TwoPhaseFlowWithPPMaterialProperties::getGasViscosity(
     return _gas_viscosity->getValue(vars);
 }
 
-Eigen::MatrixXd const& TwoPhaseFlowWithPPMaterialProperties::getPermeability(
+Eigen::MatrixXd TwoPhaseFlowWithPPMaterialProperties::getPermeability(
     const int material_id, const double t,
     const ParameterLib::SpatialPosition& pos, const int /*dim*/) const
 {
diff --git a/ProcessLib/TwoPhaseFlowWithPP/TwoPhaseFlowWithPPMaterialProperties.h b/ProcessLib/TwoPhaseFlowWithPP/TwoPhaseFlowWithPPMaterialProperties.h
index 21611b96e874b8aed804b0db71db16bd063f4c1d..1531e1f6cb1d6db88a6aa672bc1b9e69a6c9f00f 100644
--- a/ProcessLib/TwoPhaseFlowWithPP/TwoPhaseFlowWithPPMaterialProperties.h
+++ b/ProcessLib/TwoPhaseFlowWithPP/TwoPhaseFlowWithPPMaterialProperties.h
@@ -70,11 +70,10 @@ public:
 
     int getMaterialID(const std::size_t element_id);
 
-    Eigen::MatrixXd const& getPermeability(
-        const int material_id,
-        const double t,
-        const ParameterLib::SpatialPosition& pos,
-        const int dim) const;
+    Eigen::MatrixXd getPermeability(const int material_id,
+                                    const double t,
+                                    const ParameterLib::SpatialPosition& pos,
+                                    const int dim) const;
 
     double getPorosity(const int material_id, const double t,
                        const ParameterLib::SpatialPosition& pos, const double p,
diff --git a/ProcessLib/TwoPhaseFlowWithPrho/TwoPhaseFlowWithPrhoMaterialProperties.cpp b/ProcessLib/TwoPhaseFlowWithPrho/TwoPhaseFlowWithPrhoMaterialProperties.cpp
index c361307d6407dd9a1216bddf2ebcf51037b78652..290619c15f668abf76d64584b832afacb0b4568c 100644
--- a/ProcessLib/TwoPhaseFlowWithPrho/TwoPhaseFlowWithPrhoMaterialProperties.cpp
+++ b/ProcessLib/TwoPhaseFlowWithPrho/TwoPhaseFlowWithPrhoMaterialProperties.cpp
@@ -125,7 +125,7 @@ double TwoPhaseFlowWithPrhoMaterialProperties::getGasViscosity(
     return _gas_viscosity->getValue(vars);
 }
 
-Eigen::MatrixXd const& TwoPhaseFlowWithPrhoMaterialProperties::getPermeability(
+Eigen::MatrixXd TwoPhaseFlowWithPrhoMaterialProperties::getPermeability(
     const int material_id, const double t,
     const ParameterLib::SpatialPosition& pos, const int /*dim*/) const
 {
diff --git a/ProcessLib/TwoPhaseFlowWithPrho/TwoPhaseFlowWithPrhoMaterialProperties.h b/ProcessLib/TwoPhaseFlowWithPrho/TwoPhaseFlowWithPrhoMaterialProperties.h
index 49fca9ce8f0f7c9ce7e55a29da82462a749d91e9..cc66d0c26d5e5670ec1e66b057ad0b64d4e8ca87 100644
--- a/ProcessLib/TwoPhaseFlowWithPrho/TwoPhaseFlowWithPrhoMaterialProperties.h
+++ b/ProcessLib/TwoPhaseFlowWithPrho/TwoPhaseFlowWithPrhoMaterialProperties.h
@@ -63,11 +63,10 @@ public:
 
     int getMaterialID(const std::size_t element_id);
 
-    Eigen::MatrixXd const& getPermeability(
-        const int material_id,
-        const double t,
-        const ParameterLib::SpatialPosition& pos,
-        const int dim) const;
+    Eigen::MatrixXd getPermeability(const int material_id,
+                                    const double t,
+                                    const ParameterLib::SpatialPosition& pos,
+                                    const int dim) const;
 
     double getPorosity(const int material_id, const double t,
                        const ParameterLib::SpatialPosition& pos, const double p,