Given this simple implementation of string calculator, mid way, how would you refactor this, based on SOLID principals? and how far would you go?

StringCalculatorTests.cs

using NUnit.Framework;

namespace StringCalculatorTests
{
    [TestFixture]
    public class StringCalculatorTests
    {
        [TestCase("",0)]
        [TestCase("1",1)]
        [TestCase("1,2",3)]
        public void Add_Empty_DefaultResults(string input,int expected)
        {
            StringCalculator sc = new StringCalculator();

            int result = sc.Add(input);

            Assert.AreEqual(result, expected);
        }

    }

    public class StringCalculator
    {
        public int DEFAULT_RESULT = 0;

        public int Add(string input)
        {
            if (input==string.Empty)
            {
                return DEFAULT_RESULT;
            }
            if (input.Contains(","))
            {
                return handleMultiple(input);
            }
            return ParseSingle(input);
        }

        private static int ParseSingle(string input)
        {
            return int.Parse(input);
        }

        private int handleMultiple(string input)
        {
            string[] numbers = input.Split(',');
            return Add(numbers[0]) + Add(numbers[1]);
        }
    }
}