@Geneticsbunny "Also worth knowing that if you register as a self builder with the council you don't have to pay vat on the renovation work."
This is not correct. First off, VAT is nothing to do with the local council and secondly, renovation work is different from a conversion (and renovations would not qualify).
@WestrayWife Although, having said that, Geneticsbunny does make a fair point that you should consider the VAT position.
If you have a residential property that was converted into a non-residential building and you are now converting it back to a residential dwelling then, as long as the property has been non-residential for at least ten years, then you can reclaim VAT.
"...was turned into offices a few decades ago."
OK, no problems there then. However, you cannot live in the property at any time before the work starts, as this will reset the counter. For your claim to be valid, you must have evidence that no one has lived in the building for the entire ten year period.
Going forward, you can make a single reclaim of VAT paid, up to six months after you complete the conversion. This is done under the VAT431C Reclaim Scheme, all the details are here:
https://www.gov.uk/guidance/claim-a-vat-refund-for-a-conversion-if-youre-a-diy-housebuilder
There are different rules that apply depending on who your contractor/s is/are and who is responsible for purchasing the materials.
Any materials that you purchase yourself (whether it's tiles, baths, radiators etc) you will pay 20% VAT and will be able to reclaim that.
With any contractors that provide services and/or materials you must make them aware that this is a qualifying conversion and that they should only charge 5% VAT.
If you do not do this and they charge you the standard 20% VAT then HMRC will still only allow you to reclaim 5%.
If the builder incorrectly charges VAT at the standard rather than the reduced rate and you pay it, HMRC won’t process your claim. You’ll then be left in the position of trying to recoup the difference by asking your contractor to refund you.
Likewise, if your contractor is not VAT registered, never let them be involved in purchasing materials. You have the advantage of being able to reclaim the VAT, but an unregistered contractor cannot.
But it does get really complicated and confusing. For example, you can claim back VAT on your gas combi-boiler but not the gas cooker or hob in your kitchen (although you might possibly do so if there is a combined extractor and you keep very quiet about it!)
But Agas that also include an element of home heating, eg back boiler, are not subject to VAT - regardless of there being an extractor.
Another example, you can reclaim the VAT on wooden floors but not carpets.
It really can be very confusing indeed.
@Geneticsbunny "Also there is a thing called community infrastructure levy which you have to pay if you don't self build. It is based on the square meteage of the completed property."
This is a very relevant point. And it is here that you do actually need to register with the local council as a self builder.
Many councils do impose a Community Infrastructure Levy (CIL) payment on new dwellings (including conversions). Although some do still use the older Section 106 Agreement law to impose conditions.
CIL payments can be quite large. For example, where I live the CIL payments are between £150 and £200 per square metre for a new single dwelling. So an average three bed home of 100 sq metres they would charge between £15,000 and £20,000 and double those figures for a 200 sq metre property.
However, there is an exemption for self builders. If you occupy the property yourself for at least three years then you will not have to pay this fee.
However, there are very specific steps that you must go through to get the exemption. Please seek specific advice relevant to your own situation about this.