Love the add ons, but as vin says they need to be making at least what you would get on standard on the glass work, I am to get our normal hourly turnover + 50% for straightforward SFG and conny cleans - more for pressure washing work.
As a one man biz, it's more difficault to fit these in, but it really depends on you, I used to do add ons after the windows working until dark ( well past 7pm in summer ) and Saturdays.
Yes this was totally knackering, but easily doubled a weekly income, if not tripled and allowed be to invest in equipment,advertising etc.. And take on staff sooner than "just" window cleaning.
A little thought that kept me going some days was this ( esp. Where the customer, had a reg window cleaner who refused to do the conny roof or whatever ) I have just earnt in 1 hr cleaning this conny what takes their windy to earn in a year !
That's not being big headed, but the windy is happy collecting his £10 for 25 minutes work. And repeating that 12 times ( 12 visits, 12 lots of diesel, 12 lots of maybe waiting to get paid etc.. ) yet is not interested in that nice lump sum all in one day, cant understand it ?
Darran
Darran all respect to you as I know in last 4 years your growth has shot past mine (git 
) but until recently my thinking was like yours ref add ons and lump sums but my thinking changed when I sat back and realised I wanted to grow my business. Looking back the last 2 years have been ok for me but the add ons have slowed my growth down as they have distracted me.
Now we all only have so many hours in a working day and currently as a 1 man band I have to utilise em long term as best I can so when I realised that if I canvass for a few hours during week and every Saturday instead of doing add ons then the following is more than possible
5 hours canvassing / leaflet dropping a week will on average give me at least 2 to 3 new customers a week @ say £12 average
48 weeks x 2 new customers per week = 96 a year x £12 = £1152 x 12 months = £13824 (after a full 12 months of course)
48 weeks x 3 new customers per week = 144 a year x £12 = £1728 x 12 months = £20736 (as above)
Bearing in mind im 5 years into the job Im embarrassed that the above took me until last month to realise 
Yes takes commitment to get out there and do it every Saturday / after a days graft on the glass but just keep seeing that second van in my minds eye 
Will start rolling out my leafleting programme in April of 6000 leaflets a month which with a 0.2% will give me another 12 customers a month from leaflets but will canvass area after the drop so will bring up numbers.
Like I say you have done brilliantly with your growth so not a pop in any way shape or form where as I haven't so gonna try it differently for rest of this year with no more add ons (and sore shoulders )
I think the difference is that Darran has people working for him, therefore it makes sense with the upselling of additional services.
If I was just working on my own I would probably just stick with window cleaning, in fact I haven't done many pressure washing myself at all and not many gutter cleans either - I get the guys to do the majority of that work.
By offering additional services to window cleaning it really helps with cash flow, this cannot be underestimated. Plus you can treble your income from one customer, I think I've posted it on here before but I'll do it again:
A customer I've got is £30 4 weekly on window cleaning = £390 per year
Once a year gutters cleared, GFS cleaned = £350
Patio cleaned once a year = £310
So instead of one account bringing in £390 per year it brings in £1,050. And the pressure cleaning and gutter jobs are paid in full - thus benefitting cash flow.
And whilst the gutters or patio are being cleaned other work elsewhere is getting done.
For example last week we did 3 gutter cleaning jobs, which in total took about 6.5 hours, whilst they were getting done all the regular window cleaning work was getting done. The gutters bought in an additional £800. I wouldn't have been able to do all that by myself. The window cleaning money will take a while to drip through, but the gutter cleans have all paid.
It makes more business sense to have 300 customers with a much higher earning average than 600 customers with a lower one.
It depends on how you want to run your business really. I'm not a window cleaner who offers additional services. I run a business which provides exterior cleaning services, with window cleaning being one of them - the beauty with window cleaning is it's regular, repeat business.
Tom