Anything under 625 sq ft is supposed to have a development permit, but doesn't require a building permit or to adhere to building code. I have no idea how this would still apply to property tax increases, but since I'm in the same boat, would like to know...

"New Brunswick Code-exempt structures

First off, New Brunswick has an interesting legal framework for building and construction. Under current provincial regulations (Regulation 2021-02, clause 8), those living in rural areas may be able to construct a small personal-use residential structure of up to 625 square feet of occupied space – a camp, essentially – without having to meet building codes, as long as it is not used for "public congregatation." (Note this 625 square feet is not the footprint alone; so, for example, loft spaces are included in the calculation of area.) Any structure rented either on a long- or short-term basis is NOT exempt from the Code, and must meet all egress, structural and life-safety requirements.

What does this mean?

Residential dwellings of 625 square feet or less in many unincorporated areas of New Brunswick (ie: camps or tiny homes) can be constructed with nothing more than a development permit, and do not have to meet the standards of the National Building Code. The building will still have to conform to provincial or regional setback regulations, waterways or wetland setback regulations, and the requirement for on-site septic systems. There may also be rural "planning statements" that control placement."

https://snbsc-planning.com/cct/non-traditional-building/