Hi, You don't need a plan/drawings to sell your property.  it will be handy if you have the drawings as per the existing house condition for the future buyer should you sell it in the future. Most of the time it is a bonus for you if you have the drawings of your house as built. It wont cause any problem for the future extension because the architect will do some measure drawings if they don't have the existing house Building Plan.

However the issue here is the CCC. If you follow the law, ( yes i mean the law) all building works including renovation and extension also need a CCC (certificate of compliance and completion) to be issued by an architect when the construction is completed. Your extension was only  5/6 feet extension at the kitchen, I dont think it will be a problem unless some haters go report you.

So in case you are doing any future construction project, do remember that a CCC is a must to be issued by the architect when the construction is completed.

Thanks