It is true that the curves and corners are costly to build, so the first step in reducing your home extension cost is to keep the extension design as simple as possible. I don’t think you really can get a ‘Big home’ with a low budget. May be you can buy a large lot with a friend or family member ( preferably someone you don't mind having as a neighbor) that can be split into two smaller lots.
Some of the most appealing properties may be much larger than what you want or can afford, yet the seller may not be willing to break up a large parcel. So if you can share the cost with someone else, you may be able to pick up a great building site for a reasonable price. Also, you could choose low-maintenance building materials like vinyl or cementitious siding and metal roofing which may seem expensive at the time of installation but will pay for themselves in the long run.
Splurge only on those things that are extremely important. However, don't skimp on doors and windows as they are significant in keeping your home safe and secured. Buy the best you can afford in this area. One important point to keep in mind is that- Don't overbuild for the neighborhood. A home that is bigger and better than any others in its area will not command a fair price at resale. The assessment will be colored by the lower-priced homes around it.
A cost effective way to add more space to your home is cantilever bay. You can read more about this here
http://www.waldenhomes.ca/mini-addition.
Most extension work will attract VAT at 20 per cent on labor and materials, but if you use self-employed tradespeople who each turnover less than the threshold for VAT registration, you will not be charged this tax — saving on labor costs. Second-hand materials sold by private individuals on the internet will also be free of VAT.