James Ross Thompson seeks review of his conviction for unlawful manufacture of a controlled substance. Thompson asks this court to review three issues: (1) whether police could forcibly enter his home to execute a civil arrest warrant; (2) whether the “community care-taking function” allowed police to enter his home when there was no reason to believe someone was in danger or presented danger in the home; and (3) whether police were required to obtain his consent before searching the boathouse on the property. We conclude that when serving a civil arrest warrant, police may not forcibly enter into a home, and that in this case, a police officer’s entry into Thompson’s home to retrieve a guest’s jacket was not justified by the community caretaking function. However, when applying the “common authority” standard, Thompson fails to qualify as a co-occupant and therefore his consent was not necessary to search the boathouse.
I
Thompson lived on his parents’ property on Fox Island. He resided in a 22-foot travel trailer, owned by his parents, while his parents lived in a house on the property. Also on the property was a boathouse that had housed a boat owned by the elder Thompsons.
In June of 2000, Thompson’s father, John, wanted Thompson removed from his property because John suspected that Thompson was involved in illicit drug activity. John contacted the Pierce County Sheriff’s Office and reported that Thompson had an outstanding warrant for his arrest. Deputy Larson from the sheriff’s office testified that he *798confirmed such warrant existed and it was for Thompson’s failure to pay child support.
On June 5, 2000, Deputies Larson and Myron went to the trailer where Thompson was living to arrest him on the outstanding warrant. Upon arrival at the travel trailer, Deputy Larson announced, “[T]his is the sheriff[’]s office, I have a warrant for James[’] arrest.” Clerk’s Papers at 38; Report of Proceedings (RP) at 58. The deputies then heard movement and scuffling inside the trailer, and after waiting approximately 10 seconds, the deputies opened the door of the trailer. The deputies immediately saw Thompson and ordered him out of the trailer and to put his hands onto the trailer. Thompson was then handcuffed by Deputy Myron.
The deputies also observed that there was another individual inside the trailer and ordered him to come out also. Sund was patted down to make sure he was unarmed and told to leave the area. Before leaving, Sund told the deputies that he needed his jacket from inside the trailer. Deputy Larson entered the trailer to retrieve Sund’s jacket and to make sure that no one else was inside. While inside the trailer, Deputy Larson observed that the oven was open and in it “was a container that had white crystalline residue cooked onto it.” RP at 41. He also testified that he smelled a strong chemical similar to paint thinner. Id. at 42. Based on his experience and training, Deputy Larson was concerned that the odor he detected was methamphetamine related so he quickly left the trailer.
After leaving the trailer, Deputy Larson placed Thompson in the back of the patrol car and Deputy Myron read Thompson his Miranda1 rights. Based on the odor observed in the trailer, Deputy Larson went toward the elder Thompsons’ home to look for Sund and to inform John Thompson of his son’s arrest. Deputy Larson acknowledged that he wanted to arrest Sund because he was in the trailer where the deputy observed the suspicious items.
*799The elder Thompsons informed Deputy Larson that no one had come to the house. John Thompson then asked Deputy Larson to search the attached garage. Deputy Larson did not find anyone in the garage and asked John about the detached boathouse. The elder Thompson said that the boathouse was his, that James used it, and answered “ ‘Please do,’ ” when Deputy Larson asked for permission to look inside. RP at 45.
Deputy Larson did not find Sund in the boathouse, but he did find items that were consistent with a methamphetamine lab in a living area on the second floor. After this observation, Deputy Larson asked the elder Thompsons to sign a consent form for a search of the boathouse, which they both did. Deputy Larson did not seek Thompson’s consent either before or after the search.
At some point during his time on the property, Deputy Larson called for a team of methamphetamine lab investigators. Deputy Harms, a clandestine lab investigator, responded to the call. After conferring with Deputy Larson, Deputy Harms entered the trailer to make sure that the oven was turned off. Deputy Harms then inspected a bum barrel and a couple of burn piles outside the trailer that contained material consistent with the production of methamphetamine. He also checked the safety of a corroded propane tank that was located in front of the trailer. Finally, Deputy Harms looked inside the boathouse and observed the same items found by Deputy Larson earlier. After determining that the property appeared to be a methamphetamine lab but that it was a “fairly safe environment,” Deputy Harms secured the premises. RP at 79-80. Deputy Harms returned the next day with a search warrant to process the evidence.
Thompson was charged with one count of unlawful manufacture of a controlled substance, methamphetamine. See former RCW 69.50.401(a)(l)(ii) (1998).2 He sought to suppress the evidence obtained following his arrest. Pierce *800County Superior Court denied his motion to suppress evidence found in the trailer but concluded that Thompson’s consent was necessary before the search of the boathouse. Despite finding the search of the boathouse invalid, the trial court convicted Thompson as charged following a bench trial on stipulated evidence.
In a published decision, Division Two of the Court of Appeals affirmed Thompson’s conviction. State v. Thompson, 112 Wn. App. 787, 51 P.3d 143 (2002). Regarding the issue of forcible entry on a civil warrant, the Court of Appeals concluded that the knock and wait statute (RCW 10.31.040) could be applied to the service of such warrant because the deputies involved could not determine whether the warrant was for a criminal or civil matter and, thus, the officers had not acted unreasonably under the circumstances. Thompson, 112 Wn. App. at 795 (“[W]e decline to require officers at the scene of an arrest to anticipate the nature of any resulting court proceeding.”).
With regard to the retrieval of Sund’s jacket from the trailer, the Court of Appeals held that it was a valid exercise of the officer’s community caretaking function. Thompson, 112 Wn. App. at 797. Finally, the Court of Appeals concluded that the officers did not need to obtain the consent of Thompson to search the boathouse because the boathouse was a place “where one cohabitant might receive a visitor without the other cohabitant’s consent.” Id. at 802.
II
Knock and Wait Statute
Thompson argues that police officers should not be permitted, under the authority of RCW 10.31.040 (the “knock and wait” statute), to forcibly enter a private dwelling to serve a civil arrest warrant in a civil contempt procedure. We agree.
The “knock and wait” statute provides:
*801To make an arrest in criminal actions, the officer may break open any outer or inner door, or windows of a dwelling house or other building, or any other inclosure, if, after notice of his office and purpose, he be refused admittance.
RCW 10.31.040 (emphasis added).
The interpretation of a statute is a question of law, which is reviewed de novo by this court. State v. Tarabochia, 150 Wn.2d 59, 63, 74 P.3d 642 (2003). This court’s primary duty in interpreting any statute is to discern and implement the intent of the legislature. State v. J.P., 149 Wn.2d 444, 450, 69 P.3d 318 (2003). If the statute’s meaning is plain on its face, we are to give effect to that plain meaning as an expression of legislative intent. Tarabochia, 150 Wn.2d at 63. When the plain language is unambiguous—that is, when the statutory language has but one meaning—the legislative intent is clear, and the statute will not be interpreted otherwise. J.P., 149 Wn.2d at 450. Further, we cannot “ ‘add words or clauses to an unambiguous statute when the legislature has chosen not to include that language.’ ” Id. (quoting State v. Delgado, 148 Wn.2d 723, 727, 63 P.3d 792 (2003)).
The plain language of RCW 10.31.040 is clear. Its unambiguous language does not encompass the enforcement of civil arrest warrants. Because we cannot add words or clauses to an unambiguous statute, we are prohibited from reading into the statute “civil actions.” Thus, we presume that the legislature intended to exclude “civil actions” from RCW 10.31.040. Therefore, we hold that RCW 10.31.040 does not allow forcible entry into dwellings to execute civil warrants.
In the present case, there was a bench warrant for Thompson’s arrest for failure to appear at a show cause hearing regarding his failure to pay child support. This warrant was issued under RCW 26.18.050, which provides that a civil bench warrant may be issued in such circumstances. RCW 26.18.050(3). In light of our holding today, the deputies erred in forcibly opening the trailer door when *802executing the civil warrant. The “knock and wait” statute does not encompass the execution of civil arrest warrants.
Community Caretaking Function
It has long been held that warrantless searches are per se unreasonable under the fourth amendment to the United States Constitution. State v. Kinzy, 141 Wn.2d 373, 384, 5 P.3d 668 (2000). However, there are exceptions to this warrant requirement. Id. The State bears the burden of showing a warrantless search falls within one of these exceptions. Id.
The community caretaking function, which is divorced from the criminal investigation, is one such exception to the warrant requirement. Id. at 385. This exception allows for the limited invasion of constitutionally protected privacy rights when it is necessary for police officers to render aid or assistance or when making routine checks on health and safety. Id. at 386. Such invasion is allowed only if (1) the police officer subjectively believed that someone likely needed assistance for health or safety concerns, (2) a reasonable person in the same situation would similarly believe that there was need for assistance, and (3) there was a reasonable basis to associate the need for assistance with the place being searched. Id. at 386-87. “Whether an encounter made for noncriminal noninvestigatory purposes is reasonable depends on a balancing of the individual’s interest in freedom from police interference against the public’s interest in having the police perform a ‘community caretaking function.’ ” Kalmas v. Wagner, 133 Wn.2d 210, 216-17, 943 P.2d 1369 (1997).
The State argues that Deputy Larson was properly exercising the community caretaking function when he entered the trailer to retrieve Sund’s jacket. The State contends that if Sund were to enter the trailer on his own to retrieve his jacket, there was a risk that Sund could destroy evidence, retrieve a weapon, or even steal items that belonged to Thompson. Resp’t’s Suppl. Br. at 9. This argument is not persuasive.
*803When Deputy Larson entered the trailer to retrieve Sund’s jacket, there is no evidence in the record to indicate that Deputy Larson believed Sund was armed, his jacket contained a weapon, or that Sund would have entered the trailer to destroy evidence. Absent such beliefs, a reasonable person would conclude that there was no immediate need for assistance for health or safety concerns. Further, the need to retrieve Sund’s jacket from the trailer does not outweigh Thompson’s privacy interest in the trailer. Thus, Deputy Larson was not properly using the community caretaking function when he retrieved Sund’s jacket from the trailer.
The State fails to meet its burden of proving that retrieval of Sund’s jacket was a proper use of the community caretaking function as an exception to the warrant requirement. Therefore, evidence obtained from this entry into the trailer should have been suppressed.
Consent to Search
Another exception to the warrant requirement is consent to search. State v. Walker, 136 Wn.2d 678, 682, 965 P.2d 1079 (1998). It is the State’s burden to establish that a consent to search was lawfully given. Id. In order to meet this burden, three requirements must be met: (1) the consent must be voluntary, (2) the person consenting must have the authority to consent, and (3) the search must not exceed the scope of the consent. Id. Here there is no question that the consent to search the boathouse given by Thompson’s father was voluntary; nor is there any argument that the search that followed entry exceeded the scope of the consent. Thus, the sole issue in determining the validity of the consent here is the requirement for the person consenting to have the authority to consent.
In United States v. Matlock, 415 U.S. 164, 170, 94 S. Ct. 988, 39 L. Ed. 2d 242 (1974), the Supreme Court held that consent of an individual who possesses “common authority” over the area being searched is valid even *804though another person with whom that authority is shared is absent from the premises and therefore unable to consent. This court adopted the Matlock common authority standard in State v. Mathe, 102 Wn.2d 537, 688 P.2d 859 (1984). To establish lawful consent by virtue of common authority: (1) “a consenting party must be able to permit the search in his own right” and (2) “it must be reasonable to find that the defendant has assumed the risk that a co-occupant might permit a search.” Id. at 543-44. The Mathe court further elaborated on this standard by stating that the two elements are closely intertwined. Id. at 544 n.l. “If a person [consenting to the search] has joint control over an area, it may be proper to presume that the defendant reasonably assumes the risk that the joint control may be authorized to allow a search.” Id.
In Mathe, the defendant and his girl friend rented two bedrooms in a landlord’s home. Id. at 539. The landlord consented to the police search of his house in which the defendant—a burglary suspect—and his girl friend were found in one of the bedrooms. Id. The bedroom was exclusively used by the defendant, and the defendant paid rent for the use of the bedroom. Id. at 541. The landlord neither used nor had possessions stored in that room. Id. This court found that in those circumstances the landlord could not consent to a search because the tenant was in undisputed sole possession of the premises. Id. at 544. Therefore, the landlord could not be deemed a co-occupant under the common authority standard and he was not able to permit a search in his own right.
The consent rule enunciated in Matlock, and adopted by Mathe, applies to the validity of the consent of one holding common authority with an absent, nonconsenting individual. However, in State v. Leach this court held that if “the cohabitant be present and able to object, the police must also obtain the cohabitant’s consent.” 113 Wn.2d 735, 744, 782 P.2d 1035 (1989) (emphasis added). In Leach, the defendant and his girl friend ran a travel agency together. Although the defendant was the owner and operator of the *805agency, his girl friend had a key to the office, performed minor tasks for the agency, her name appeared on the lease of the premises, and her name also appeared on business cards as an “owner.” The girl friend informed the police that the defendant was responsible for the rash of burglaries that occurred in the complex where the travel agency was located. She escorted the police into the agency’s office using her key. The defendant was present when police arrived and was placed under arrest. During their search, the police discovered stolen property linked to the reported burglaries. This court held that the search was improper because the police should have obtained the defendant’s consent since he possessed at least equal control over the premises and was present at the time of the search. Id.
This issue was further examined in Walker where the nephew of a married couple was caught with a bag of marijuana at school. The nephew told police that he lived with his aunt and uncle and obtained the marijuana from their home. 136 Wn.2d at 679. He also told police that more marijuana could be found in the home. Id. The aunt signed a consent form to search the home and accompanied police to her home. Id. at 680. Before the police entered the home, the uncle arrived. Id. at 680-81. Without obtaining his consent, the police entered the home and found additional marijuana. Id. at 681. This court held that while the consent was valid against the aunt, it was not valid against the uncle since he was a co-occupant, with equal control over the premises, and had not given his consent. Id. at 686.
Given the evidence at hand, under the common authority standard as enunciated in Matlock and Mathe we conclude that Thompson was not a co-occupant of the boathouse with equal control over those premises and, that unlike the circumstances in both Leach and Walker, his consent was not required to validate the search.
To qualify as a co-occupant, it must be shown that Thompson had equal control over the premises with his parents, i.e., that he would have been able to permit the search in his own right. To be able to permit a search in his *806own right, it must be established that Thompson had joint access or control of the boathouse for most purposes. See Matlock, 415 U.S. at 172 n.7. The record does not support such a conclusion. The boathouse was on property owned by Thompson’s parents. Thompson was living on another part of his parents’ property in a travel trailer that was also owned by them. RP at 11, 45. He did not pay rent to his parents, and as testimony proved, he neither occupied the boathouse nor was it available to him for his exclusive use. RP at 16-17, 45. Although his parents allowed Thompson to store items in the boathouse, his parents did as well, and there is no evidence in the record to show that Thompson was ever in exclusive control of the boathouse. Thompson’s use of the boathouse was clearly dependent upon the permission of the owners, i.e., his parents. Thus, while Thompson and his parents each had access to the boathouse, his right to access, as a nonoccupying nonowner, was subordinate to his parents. Therefore, under the common authority standard, Thompson does not qualify as a co-occupant who had equal access and control over the boathouse.
By contrast, it is clear that the parties in Leach and Walker were co-occupants since they possessed equal control and access over the searched premises. Although in Leach the defendant was the employer of the girl friend and was the sole owner of the travel agency, the couple held her out to be “co-owner” of the premises, and she was a signatory to the lease of the searched premises. Therefore, to an observer, the defendant and his girl friend had joint control and access over the searched premises. In Walker, joint control and access was apparent because the husband and wife defendants were owners of the searched home and the room being searched was their joint bedroom. Here, however, Thompson does not enjoy such status. Thompson, an adult son, was living on a portion of his parents’ property rent free. This type of relationship does not equate to Thompson possessing joint control over all his parents’ property. As stated above, his access to the boathouse was *807contingent upon his parents’ permission. Because he lacked the authority to do so, Thompson could not have permitted the search in his own right.3 Therefore, we find that Thompson did not possess common authority over the boathouse and that his consent was not necessary to the validity of the search.
Suppression of Evidence
Following the elder Thompsons’ consent, Deputy Harms entered the boathouse and conducted an initial scan of the premises for officer safety. RP at 77. In plain view, he observed the following: small jars containing different types of liquid; coffee filters; mason jars containing coffee filters and “white powder”; electric hot plates; tubing; rock salt; pie plates with white residue; bottles of liquid ammonia; glassware; starting fluid; an empty HC1 gas generator; five-gallon buckets containing unknown fluid, one with a syringe floating in it; a bag containing “rough” pseudo-ephedrine tablets; and an empty bottle of isopropyl alcohol. RP at 78. Based on his experience, Deputy Harms suspected that the boathouse was being used as a methamphetamine lab, discontinued further observation, and sought a search warrant. Id. at 79.
Deputy Harms also observed certain items in open view in other parts of the property. “The mere observation of that which is there to be seen does not necessarily constitute a search.” State v. Seagull, 95 Wn.2d 898, 901, 632 P.2d 44 (1981). It is clear that the police with legitimate business may enter areas of curtilage which are impliedly open. In so doing, the police are free to use their senses. Id. Here, Deputy Harms observed a propane tank with a bluish corrosion on the valve near the front of Thompson’s trailer. Deputy Harms testified that such a corrosion indicates the tank had been illegally used to store anhydrous ammonia, which is a necessary chemical used in the production of *808methamphetamine. Deputy Harms also found charred blister packs from pseudoephedrine packaging in a burn barrel located on the property near the trailer. Deputy Harms further observed other burn piles on the property which contained stripped lithium battery pieces and some empty bottles of pseudoephedrine that had the bottoms removed. Both lithium metal and pseudoephedrine are main ingredients in methamphetamine production. Deputy Harms was able to view all of these items from a lawful vantage point.
The items discovered in the boathouse following a lawful consent to search by Thompson’s father together with the items found in open view on the property clearly established probable cause for the search warrant independent of items discovered in Thompson’s trailer. Therefore, the evidence obtained through the valid search warrant should not have been suppressed at trial. See State v. Coates, 107 Wn.2d 882, 888, 735 P.2d 64 (1987) (excluding tainted evidence, independently found evidence established probable cause for the search warrant).
Ill
Thompson’s conviction was based, at least in part, on evidence found within the trailer—evidence we here conclude is inadmissible. This constitutional error may be considered harmless if we are convinced beyond a reasonable doubt that any reasonable trier of fact would have reached the same result despite the error. State v. Brown, 140 Wn.2d 456, 468-69, 998 P.2d 321 (2000). To make this determination, we utilize the “overwhelming untainted evidence” test. State v. Smith, 148 Wn.2d 122, 139, 59 P.3d 74 (2002). “Under this test, we consider the untainted evidence admitted at trial to determine if it is so overwhelming that it necessarily leads to a finding of guilt.” Id.
In this case, evidence from the boathouse which should have been considered by the trial court was not admitted. Therefore, there is no other untainted evidence upon which *809we can rely to conclude that Thompson’s conviction should be affirmed. Therefore, it is impossible for us to find that the error in this case was harmless.
Because the introduction of the evidence found in the trailer does not constitute harmless error, we vacate Thompson’s conviction for unlawful manufacture of a controlled substance and remand for a new trial consistent with this opinion.
Alexander, C.J., and Johnson, Madsen, Ireland, and Owens, JJ., concur.
Miranda v. Arizona, 384 U.S. 436, 86 S. Ct. 1602, 16 L. Ed. 2d 694 (1966).
This statute was recently amended. However, the substance of the statute is intact. Laws of 2003, ch. 53, § 331.
Since it is clear that Thompson cannot permit a search of the boathouse in his own right, it is unnecessary to examine the second prong of the common authority standard.