The works may require Planning Permission or they may be a permitted development. Building Regulation approval will certainly be required.
If you are attached to another property(s), or are excavating within 3 metres of another property or structure and below the level of its foundation then the Party Wall etc Act 1996 is likely to apply.
KMASS can assist in advising you what is required and they have the required skills and expertise to guide you through these processes. If you have not already chosen a builder, they can also obtain quotations for the works on your behalf and administer the contract between yourself and the builder.
If you have obtained a quotation from a builder for works prior to the preparation of a set of drawings that are suitable for Building Regulation submission you should be aware that the quotation is unlikely to reflect the final cost of the works because the builder will have estimated foundation depths, steel work requirements etc. and costs are likely to change when the actual detailed requirements are set out.
Similarly, without a detailed set of Building regulation drawings and calculations it is usually not possible to compare builders prices because they will have each estimated different requirements in the absence of a definitive design.