If you used a credit card to pay for a job, it's worth looking into whether you can get your money back using Section 75 of the Consumer Credit Act. This applies if the job cost between £100 and £30,000. If you paid with a debit or prepaid card, or if the cost exceeded the limits of Section 75, you can try filing a chargeback request. This is not legally binding, but it's worth exploring.
You have 120 days from when the problem is detected to file a claim. If the amount is less than £10,000 in England and Wales or £5,000 in Scotland and Northern Ireland, you can take your case to small claims court. If you need help, contact the Citizens Advice consumer helpline on 0808 223 1133. You can also use an online form. If there have been delays in completing the construction works or if the works are defective, you must file a written complaint.
You must require your builder to comply with the contract and complete the work and fix all defects within a reasonable period. If this doesn't happen, you can cancel the contract. If your complaint has not been resolved, consider contacting any trade association or professional body to which the builder belongs. The builder is also obligated to correct all major structural defects at no cost to the purchaser for 8 years after completion of the work.
If you have already made some payments to your original builder (assuming that you paid him no more than for the work he did), your claim will be the difference between the amount paid to the new builder to complete the work and the amount you would have paid to your original builder, plus the cost of repairing any defects. If your builder doesn't respond or doesn't meet your demands following your complaint, it may be time to seek legal advice before considering writing to cancel the contract. According to the NHBC scheme, your builder guarantees that he will correct any defects caused by him and anything else that does not meet the required level of work.