A certified plot plan is a plot plan that has been stamped by a registered land surveyor or civil engineer. A certified plot plan shows the boundaries of your property and where the primary building(s) and other accessory structures are located on the property. They may also show easements for things such as sewer, utilities, right-of-ways, etc. or perhaps wetland boundaries.
The Town of Bedford's zoning bylaws require, among other things, that buildings or accessory structures be setback a certain distance from the property lines depending on what Zoning District your property is in. A certified plot plan is needed to verify that the proposed new building, addition or accessory structure meets the minimum setback requirements under the zoning bylaws. It also may show where your easements, if any, are located so you don't accidentally build within the easement.
For new dwellings, the owner is responsible for hiring a registered land surveyor or civil engineer to perform an instrument survey of the property and certify the location of the new dwelling and any other accessory structures within the boundary lines to make sure they comply with the setback requirements and any other applicable requirements found in the BZBL. For existing dwellings, although the Code Enforcement Office is not required to provide certified plot plans, there may be cases where a certified plot plan can be found in the property file from a previous project. At the discretion of the Building Official, if the certified plot plan is to scale and accurately depicts the current footprint/location of the primary dwelling and all accessory structures then, in some cases, the homeowner may request a copy of the certified plot plan from the file and use it to locate the proposed addition and/or the accessory structure, to scale, on the existing certified plot plan. If there is no certified plot plan found in the property file then the homeowner is responsible for hiring a registered land surveyor or civil engineer to perform an instrument survey of the property and certify the location of the existing dwelling as well as the proposed addition(s) and/or any other accessory structures within the boundary lines to make sure they comply with the setback requirements and any other applicable requirements found in the BZBL.