| Hi
Its important to be professional and look professional in the work place. Perhaps you could add something different in the dresscode scenario from time to time so that you are not deemed as the strict and stiff person. For example dresscode of the week.....maybe a certain color. Anyone who has worn something different from the agreed dresscode of the week (eg.casual wear or different color) will treat everyone else to some goodie - sweets, chocs etc). You could even introduce the best dressed person of the week to make the whole environment fun and tie that in with their appraisals. As mentioned above by Shikha, as an HR person, you need to buy in your internal customers so that they find it easier to approach you...and dont forget the discussion with your seniors.
A communication meeting with all employees about office ettiquette and dress code in these lines will go a long way. Let them come up with ideas and take the ownership of being "well dressed". Everyone is an adult and they should behave thus and be treated thus. You can be well dressed yet comfortable. End of the day, everyone wants to look good and feel good. Work towards it.
As far as weather conditions are concerned, they can always carry a change of footwear with them for outdoors.
Its not easy to change people's mindset at the blink of an eye....so it will take time....but the key is self realization which they will have to work towards with your help. | 
