Before undertaking any construction you need to know something about the planning process: it's long! How long? At best it will take 8 weeks. Add to this another 2-6 weeks for the design and you are looking at 3-4 months before you can start anything; that's the best-case scenario.
It's not that uncommon for a project to be dragged for more than a year with the planning officer asking for changes or studies along the way; or because the project/extension is too big and you have to do it in stages, otherwise you'll never get an approval on the desired scheme.
If you have in mind to start the building works in spring, my advice would be to start discussions with an architect or designer in autumn, and usually, by the time the good weather comes you have everything ready: planning, construction drawings, structural calculations and building control planning approval. On top of that, all the costings should be clear and final to avoid any surprises during the construction.
It is possible to have a construction site during winter but because the weather is unpredictable delays will happen, thus increasing the cost. Also. if the temperature is too low workers can't stay outside for too long and if it's under 5 Celsius you can't pour normal concrete.
Plan ahead for the unexpected, start the designing process around 6 months before the construction works, thus you stand a better chance not to be delayed. Getting in touch with an architect in Autumn for a consultation may be the best option to start your construction in Spring. And being ready early spring can be a cost saving in contracting a builder before the demand rises.