What exactly is the rule?
On a single project, one party may not both assess and remediate the mold. The rule comes from Texas Occupations Code Chapter 1958 and the Texas Department of Licensing and Regulation's rules, and it applies to houses, shops and offices alike, with the school district exception.
The rule concerns the project, not only the company. A firm that holds a remediation license should not also write the assessment for the same job, and a consultant should not carry out the removal it has assessed. The homeowner should check which party is doing which part.
The state's own licensing pages set out the detail. The rule is stated in plain terms there, and it is worth reading before you sign a contract, because it decides the structure of the job.
How do I check that the rule is being followed?
Ask for the names and license numbers of the assessor and the remediation contractor, check each license with TDLR, and confirm that they are different companies on your project. Keep the assessment and the remediation records in separate paper, even if they are filed together.
Write down the names on each document. If the same name appears as assessor and remediator, stop and ask the firm to explain before you go on. A firm that cannot explain it is a reason to look elsewhere.
A post-remediation check, where the assessment calls for one, is carried out by the assessor. If the check is done by the contractor who did the work, the separation has not been kept.
What if a single firm offers to do both?
On a non-school project, the firm should be told it cannot do both on the same project, and the homeowner should find a separate licensed party for the other part. A firm that offers to do both without raising the rule is not describing the Texas requirement correctly.
The easiest way to handle this is to hire the assessor first and let the assessor name the scope. The remediation contractor then bids against that scope, and the homeowner can compare bids on the same basis.
A firm may legitimately do other water damage work on the same building, such as drying and repair, while a separate licensed party handles the mold. The point of the rule is the mold assessment and remediation on the project, not the whole job.
Does the separation add to the cost?
It can add a step, because two parties are involved. It also gives an independent scope, which often makes the work easier to price and easier to check. The cost is explained in writing before any work starts, and this site does not publish a figure for either part.
The homeowner pays for the assessment and, separately, for the remediation. The two quotes should be easy to compare, because the remediation quote follows the assessment's scope.
The extra step is usually small beside the cost of work done to the wrong scope. Ask each party to explain its quote in writing, so you know what each part covers.